Change float content order depending on its position selected by LaTeX's float mechanism

Question

I'm (still) going to develop a class that will use wide margins and put some contents into it. In contrast to tufte-book, which sets the positioning parameters for floats as htbp by default, I would like to allow true floats. Furthermore, I'd like to offer an environemtn which a) floats and b) puts its caption (in the margin) either above or below the figure, depending on the positioning selected by the float mechanism. While I can handle this if a position is provided (the optional argument of figure or table), I'd like to do the same if no optional argument was specified. Since the content and hence the height of the figure is the same for both cases, it would not change the positioning of the figure.

I'm quite sure that I have read about this some years ago, but I don't find a hint via google. For the MWE, I omit the positioning in the margin but simply want to output the caption below or above the figure's content.

\documentclass{article}
\usepackage[marginparwidth=7cm,left=1.5cm,right=1cm,includemp]{geometry}
\usepackage{lipsum}
\makeatletter
\def\myt{t}\def\myp{p}\def\myb{b}
\newcommand*\widefigure[3][]{%
  % #1: Position
  % #2: Figure
  % #3: Caption
  \def\@tempa{#1}%
  \begin{figure}[#1]%
    \rlap{%
      \begin{minipage}{\dimexpr\textwidth+\columnsep+\marginparwidth}%
        \ifx\@tempa\myb
          \caption{#3}#2%
        \else\ifx\@tempa\myt
          #2\caption{#3}
        \else\ifx\@tempa\myp
          \caption{#3}#2%
        \else
         % What if no position is specified?
        \fi\fi\fi
      \end{minipage}}
  \end{figure}%
}
\begin{document}
\widefigure[b]{\rule{\textwidth}{5cm}}{foo}
\lipsum[1-4]
\widefigure[t]{\rule{\textwidth}{5cm}}{foo}
\lipsum[2]
\widefigure{\rule{\textwidth}{5cm}}{foo}
\lipsum[7]
\end{document}