\ifnum\contextmarkmode=4
\def\dostartoverprint% yes... one of the category 'asking for trouble' :D
  {\ctxlua{overprints.enabled=true}}
\def\dostopoverprint%
  {\ctxlua{overprints.enabled=false}}
\fi

\starttext

\starttextoverprint Overprint \stoptextoverprint % fails in both marks
\dostartoverprint Overprint \dostopoverprint % fails in mkiv only

\stoptext