% \unexpanded\def\myrule{\blackrule[width=10cm,color=red,height=1pt,depth=0pt]}
% \setupnote[footnote][rule=command,rulecommand=\myrule]

\startuniqueMPgraphic{noterule}{width,color,line}
    draw origin -- (\MPvar{width},0)
        withcolor \MPvar{color}
        withpen pencircle scaled \MPvar{line}
        dashed evenly ;
\stopuniqueMPgraphic

\setupnote[footnote][rule=command,rulecommand=\uniqueMPgraphic{noterule}{width=7cm,color=red,line=1pt}]

\starttext

    test \footnote{test}

\stoptext
