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