% macros=mkvi

\setuppagenumbering[location=footer]

\startusableMPgraphic{NumberHead}
     draw outlinetext.f
         ("\bf\namedheadnumber{chapter}")
         (withcolor "lightgray")
         ysized 50pt ;
\stopusableMPgraphic


\unexpanded\def\processMPheadnumber#1%
  {\useMPgraphic{NumberHead}}

\setuphead
   [chapter]
   [command=\HeadTitle,
    headstyle=\ss,
    numbercommand=\processMPheadnumber]


\unexpanded\def\HeadTitle#1#2%
  {\framed
    [frame=off,
    bottomframe=on,
    width=broad,
    align={broad,nothyphenated,left}]
    {#1\blank[nowhite]#2}}

\starttext

\dorecurse{6}{\startchapter
  [title={Fist chapter}]
  \input knuth
\stopchapter}

\stoptext

Using TL18 on Win64 it produces a large chapter number on the right side above the chapter name -- I do not know what it should look like, so I won't say it works. It clearly fails with the 2018-07-13 beta, producing an odd graphic where the MP-generated digit should be:

and fails in the same manner with the 2018-07-17 beta.

--
Rik