I'm writing a book using a tufte-like style but implemented using memoir. (I don't want to use tufte itself as memoir has lots of nice features.) Thus, I have a relatively narrow textwidth, use marginnotes, etc. There is one feature I would like to implement but do not know how, and that is to have a fullwidth figure with the caption below the figure, in the margin. The memoir side caption (MWE below) puts the figure to one side, and the regular figure environment puts the caption below the figure. I would like to have a figure caption below the figure, but in the margin, as illustrated below. Mimicking the tufte fullwidth environment is not of itself sufficient. If anyone has suggestions or, even better, some working code I'd be grateful.
\documentclass[10pt, a4paper,openright, twoside]{memoir}
\usepackage[demo]{graphicx}
\usepackage{kantlipsum}
%
\begin{document}
\kant[1]
\begin{figure}
\setlength{\sidecapraise}{-5cm} % manual adjustment of figure caption position
\begin{sidecaption}{\kant[2]}[label]
\includegraphics[width=1.2\textwidth,height=0.5\textwidth]{dummy}
\end{sidecaption}
\vskip -6cm % manual adjustment of position of main text below figure
\end{figure}
\end{document}