I'm interested in changing how floats are formatted depending on whether they are placed at the top, bottom, or middle of a page, or on a float page. One package I am aware of that does something like this is tufte-latex, which places hanging captions in the margin below full-width figures if the figures are at the top or middle, but above if the figures are at the bottom. Looking at the code for tufte-latex, though, this is a pretty rudimentary check (as near as I can tell, the caption is placed on top exactly when the only placement specifier is "b"). And when the floats are put on a float page, the hanging captions don't look right.
Ideally, I'd like to be able to tell where a float has actually been placed and do something like
\if \finalfloatplacement t%
% hang caption in margin below float
\if \finalfloatplacement h%
% same as 't'; hang caption in margin below float
\if \finalfloatplacement b%
% hang caption in margin above float
\if \finalfloatplacement p%
% center caption below float
Presumably this would take a couple of LaTeX runs to iron out the positions.