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*. I have tried 
spacebefore=\vskip but it doesn't seem to do anything. Another option 
was to use default caption location (below the figure) and add \hspace 
before figure placement.
None of these work as I am not able to make spacebefore do anything.

*MWE*

\setuplayout[width=5cm, height=13cm,rightmargin=5cm]

\setupexternalfigures[option=empty]
\setupfloat[figure][location=inner]
%
\setupcaption
   [figure]
    [width=5cm,
     location={rightmargin,low},
     spacebefore=big]

%\setupcaption
   %[figure]
   %[width=5cm]

\starttext
   \showframe
   \placefigure{A small but important
   caption.}{\externalfigure[cow][width=10cm]}
\stoptext