This answer uses a different mechanism, so I am posting it as a separate answer rather than editing the previous one. This is not a complete answer, but a proof of concept that works if the phrase is split across lines only once. It is possible to extend it so that it works if the phrase is split across multiple lines and/or multiple pages, but that requires more code.
The main idea is to use the anchoring mechanism of ConTeXt (similar to the \Tikzmark macro), record the beginning and end of the phrase, and use that the draw the background. Since the actual code of drawing the background is a bit messy, let me start by the simple task of underlining the phrase so that the underline splits across lines.
\definecolor[lightblue][b=1,r=0.5,g=0.5]
\unprotect
\newcount\c_placeholder_n
\def\placeholder#1%
{\global\advance\c_placeholder_n\plusone
\startpositionoverlay{text-1}% The layer just below text
\setMPpositiongraphicrange
{b:placeholder:\the\c_placeholder_n}%
{e:placeholder:\the\c_placeholder_n}%
{mpos:placeholder}%
{self=placeholder:\the\c_placeholder_n,
linewidth=1bp,
linecolor=red,
color=lightblue,
radius=0.5\lineheight}%
\stoppositionoverlay
\bpos{placeholder:\the\c_placeholder_n}%
#1%
\epos{placeholder:\the\c_placeholder_n}}
\protect
% This is a simple proof of concept. I assume that there is at most one line
% break, and no page break between the text.
\startMPpositionmethod{mpos:placeholder}
\startMPpositiongraphic{mpos:placeholder}{linecolor,linewidth,color,radius}%
begingroup;
newnumeric x,y, w, a_x, a_y, a_h, a_d, b_x, b_y, b_h, b_d, r ;
(x,y) = \MPxy\textanchor; % The overlay is anchored at (x,y).
% So, we have to subtract (x,y) from each point.
(a_x,a_y) = \MPxy\MPbself; % The localtion of the beginning of the line.
% Height and depth at the beginning of the line
a_h := \MPh\MPbself; a_d := \MPd\MPbself;
(b_x,b_y) = \MPxy\MPeself; % The location of the end of the line
% Height and depth at the end of the line
b_h := \MPh\MPeself; b_d := \MPd\MPeself;
w := \MPw{\textanchor}; % equal to line width for split words.
r := \MPvar{radius};
newpath p;
drawoptions (withpen pencircle scaled \MPvar{linewidth} withcolor \MPvar{linecolor});
% Note that the line is drawn at the baseline of the word.
% If you subtract a_d to the y-coordinate, then the line will
% be drawn so that it is below the depth of the characters (i.e., below the
% 'p' in placeholder).
% If you add a_h to the y-coordinate, then the line will be drawn so
% that it is at the top of the characters.
if a_y = b_y : % Word is not split
p := (-x + a_x, -y + a_y ) -- (-x + b_x, -y + b_y) ;
draw p;
else : % word is split across lines
% Draw the first part
p := (-x + a_x, -y + a_y) -- (w, -y + a_y);
draw p;
% Draw the second part
p := (0, -y + b_y) -- (-x + b_x, -y + b_y);
draw p;
fi
endgroup;
% Now, to complete the solution, all you need to do is to constuct an
% appropriate metapost shape.
\stopMPpositiongraphic
\MPpositiongraphic{mpos:placeholder}{}%
\stopMPpositionmethod
\setuppapersize[A5]
\starttext
A short \placeholder{placeholder} in a line
A very long line that splits across lines at the word \placeholder{place\-holder~word} and continues after that.
\stoptext
which gives
The rest is just a matter of details to tweak the MP position method so that it draws the required shape. For example, if you change it to
\startMPpositionmethod{mpos:placeholder}
\startMPpositiongraphic{mpos:placeholder}{linecolor,linewidth,color,radius}%
begingroup;
newnumeric x,y, w, a_x, a_y, a_h, a_d, b_x, b_y, b_h, b_d, r ;
(x,y) = \MPxy\textanchor; % The overlay is anchored at (x,y).
% So, we have to subtract (x,y) from each point.
(a_x,a_y) = \MPxy\MPbself; % The localtion of the beginning of the line.
% Height and depth at the beginning of the line
a_h := \MPh\MPbself; a_d := \MPd\MPbself;
(b_x,b_y) = \MPxy\MPeself; % The location of the end of the line
% Height and depth at the end of the line
b_h := \MPh\MPeself; b_d := \MPd\MPeself;
w := \MPw{\textanchor}; % equal to line width for split words.
r := \MPvar{radius};
newpath p;
drawoptions (withpen pencircle scaled \MPvar{linewidth} withcolor \MPvar{linecolor});
if a_y = b_y : % Word is not split
p := (-x + a_x + r/2 , -y + a_y - a_d) {left}
.. {up}(-x + a_x - r/2, -y + a_y + (a_h - a_d)/2 ){up}
.. {right}( -x + a_x + r/2, -y + a_y + a_h)
-- (-x + b_x - r/2, -y + b_y + b_h) {right}
.. {down} (-x + b_x + r/2, -y + b_y + (b_h - b_d)/2 ) {down}
.. (-x + b_x - r/2, -y + b_y - b_d)
-- cycle;
fill p withcolor \MPvar{color};
draw p;
else : % word is split across lines
% Draw the first part
p := (-x + a_x + r/2 , -y + a_y - a_d) {left}
.. {up}(-x + a_x - r/2, -y + a_y + (a_h - a_d)/2 ){up}
.. {right}( -x + a_x + r/2, -y + a_y + a_h)
-- (w , -y + a_y + a_h)
-- (w , -y + a_y - a_d)
-- cycle;
fill p withcolor \MPvar{color};
draw p;
% Draw the second part
p := (0, -y + b_y - b_d)
-- (0, -y + b_y + b_h)
-- (-x + b_x - r/2, -y + b_y + b_h) {right}
.. {down} (-x + b_x + r/2, -y + b_y + (b_h - b_d)/2 ) {down}
.. (-x + b_x - r/2, -y + b_y - b_d)
-- cycle;
fill p withcolor \MPvar{color};
draw p;
fi
endgroup;
% Now, to complete the solution, all you need to do is to constuct an
% appropriate metapost shape.
\stopMPpositiongraphic
\MPpositiongraphic{mpos:placeholder}{}%
\stopMPpositionmethod
you get
As I said earlier, it is possible to add support for multiple lines (similar to the MkII version of the underline macro, but that assumes that the interline spacing is constant; an assumption that may not always be true). The underline macro in MkIV uses a completely different attributes based mechanism to draw the underline. It may be possible to tweak that to get rounded corners, but I don't understand that code well enough to suggest how to do that.
EDIT: The version below takes care of the placeholder splitting across pages.
\startMPpositionmethod{mpos:placeholder}
\startMPpositiongraphic{mpos:placeholder}{linecolor,linewidth,color,radius}%
begingroup;
newnumeric x,y, w, a_x, a_y, a_h, a_d, b_x, b_y, b_h, b_d, a_p, p_h, r ;
(x,y) = \MPxy\textanchor; % The overlay is anchored at (x,y).
% So, we have to subtract (x,y) from each point.
(a_x,a_y) = \MPxy\MPbself; % The localtion of the beginning of the line.
% Height and depth at the beginning of the line
a_h := \MPh\MPbself; a_d := \MPd\MPbself;
(b_x,b_y) = \MPxy\MPeself; % The location of the end of the line
% Height and depth at the end of the line
b_h := \MPh\MPeself; b_d := \MPd\MPeself;
% Page number of the anchors
a_p := \MPp\MPbself; b_p := \MPp\MPeself;
w := \MPw{\textanchor}; % equal to line width for split words.
r := \MPvar{radius};
newpath p;
drawoptions (withpen pencircle scaled \MPvar{linewidth} withcolor \MPvar{linecolor});
if a_y = b_y : % Word is not split
p := (-x + a_x + r/2 , -y + a_y - a_d) {left}
.. {up}(-x + a_x - r/2, -y + a_y + (a_h - a_d)/2 ){up}
.. {right}( -x + a_x + r/2, -y + a_y + a_h)
-- (-x + b_x - r/2, -y + b_y + b_h) {right}
.. {down} (-x + b_x + r/2, -y + b_y + (b_h - b_d)/2 ) {down}
.. (-x + b_x - r/2, -y + b_y - b_d)
-- cycle;
fill p withcolor \MPvar{color};
draw p;
else : % word is split across lines
% If the beginning anchor is on the current page, draw the first part
if a_p = RealPageNumber :
p := (-x + a_x + r/2 , -y + a_y - a_d) {left}
.. {up}(-x + a_x - r/2, -y + a_y + (a_h - a_d)/2 ){up}
.. {right}( -x + a_x + r/2, -y + a_y + a_h)
-- (w , -y + a_y + a_h)
-- (w , -y + a_y - a_d)
-- cycle;
fill p withcolor \MPvar{color};
draw p;
fi
% The the end of the anchor is on the current page, draw the second
% part
if b_p = RealPageNumber :
p := (0, -y + b_y - b_d)
-- (0, -y + b_y + b_h)
-- (-x + b_x - r/2, -y + b_y + b_h) {right}
.. {down} (-x + b_x + r/2, -y + b_y + (b_h - b_d)/2 ) {down}
.. (-x + b_x - r/2, -y + b_y - b_d)
-- cycle;
fill p withcolor \MPvar{color};
draw p;
fi
fi
endgroup;
\stopMPpositiongraphic
\MPpositiongraphic{mpos:placeholder}{}%
\stopMPpositionmethod
\inframed[...,, strut=no, ...]{...}.\inframetogether with the text is shifted down (~2mm).