\defineoverlay
  [frames]
  [\positionoverlay{frames}]

\setupbackgrounds
  [text]
  [background=frames]

\startMPpositiongraphic{frame}
    if unknown context_apos : input mp-apos.mpiv ; fi ;
    \MPgetposboxes{\MPvar{self}}{\MPanchorid}
    if nofposboxes = 1 :
        draw posboxes[1] ;
    fi ;
\stopMPpositiongraphic

\startpositionoverlay{frames}
    \setMPpositiongraphic{one}{frame}
    \setMPpositiongraphic{two}{frame}
\stoppositionoverlay

\starttext
    This \hpos{one}{word} should be \hpos{two}{framed}.
\stoptext
