Well, if you're open to a slight change in the syntax and some hackery, you can create frankenstein nodes by abusing TikZ's label
, pin
and append after command
options.
The rectangle split
shape already is close to what you want. However, it doesn't allow using a different line style for the separator. We can, however, prevent the separator from being drawn at all and provide our own path. This is done using an edge operation in the append after command
.
The attached rectangle can be drawn by using the label
option. Properly positioning it involves a shift so that the borders overlap each other, which is done with the outer xsep
option. To allow references to this new node's anchors, we also give it a name which is constructed from the name of the main node.
Here's a MWE:
\documentclass[border=1mm,tikz]{standalone}
\usetikzlibrary{shapes.multipart}
\tikzset{entry/.style={
% appearance settings for the main node
draw,
rectangle split,
rectangle split parts=2,
rectangle split draw splits=false,
% draw the small attached node
label={[name=label of \tikzlastnode,anchor=north west,draw,outer xsep=-.5\pgflinewidth]north east:#1},
% draw the split using a dashed line
append after command={(\tikzlastnode.text split west) edge[draw,dashed] (\tikzlastnode.text split east)},
}}
\begin{document}
\begin{tikzpicture}
\node[entry=2] (myentry) at (0,0) {1001 \nodepart{second} 1010};
% examples of how we can refer to coordinates of this monster
\node[coordinate,pin=4:a] at (label of myentry.south east) {};
\node[coordinate,pin=4:b] at (myentry.south east) {};
\end{tikzpicture}
\end{document}

A caveat is that you can't use the anchors of the extra node while the construct is being placed. Also, the contents of this node are an option to the style instead of a real node part.