On 2013–01–20 Devendra Ghate wrote:

> I am using a layout with wide right margin. For floats that are
> \textwidth wide, I can place captions in the margin
> using**\setupcaption[location=rightmargin,high/low].
> 
> However, for figures that are
> (*\textwidth+\marginwidth+\margindistance*) wide, I would like to
> *place the caption in the margin below the figure*.

For a recent project I had the similar requirements. I did not find
a proper solution. I attached an example of the code I used.
However, it has several issues:

1) Margin floats and texts overlay and need manual adjustment.
2) The baseline of the float description does not match the baseline
   of the main text (fixed using  manual trial and error). I used an
   ugly hack (toffset=7.5ex) to align the description vertically.
3) The \startplacesmartfigure is not very robust. My notes say “Do
   not indent \startplacesmartfigure”. I don't recall what the exact
   issue was.
4) Probably many more that I haven't discovered, yet.

Feel free to suggest fixes for the mentioned issues.

Marco