… and I still don't get it working. I used the example from the
wiki. The problem: The frame is drawn on *every* cell. It basically
looks like a global frame=on.

Minimal example to draw two rules in a table (44 lines, seriously?!)

\startuseMPgraphic{tableborders}
  draw leftboundary   OverlayBox withpen pensquare scaled \frameddimension  {leftrulethickness} ;
  draw rightboundary  OverlayBox withpen pensquare scaled \frameddimension {rightrulethickness} ;
  draw topboundary    OverlayBox withpen pensquare scaled \frameddimension   {toprulethickness} ;
  draw bottomboundary OverlayBox withpen pensquare scaled \frameddimension{bottomrulethickness} ;

  clip currentpicture to OverlayBox
    leftenlarged   (\frameddimension  {leftrulethickness}/2)
    rightenlarged  (\frameddimension {rightrulethickness}/2)
    topenlarged    (\frameddimension   {toprulethickness}/2)
    bottomenlarged (\frameddimension{bottomrulethickness}/2) ;

  setbounds currentpicture to OverlayBox ;

\stopuseMPgraphic

\defineoverlay [tableborders] [\useMPgraphic{tableborders}]

\startsetups tableborders

\setupTABLE
  [         background=tableborders,
                 frame=off,
      backgroundoffset=0pt,
     leftrulethickness=\framedparameter{rulethickness},
    rightrulethickness=\framedparameter{rulethickness},
      toprulethickness=\framedparameter{rulethickness},
   bottomrulethickness=\framedparameter{rulethickness}]

\setupTABLE  [row] [first] [toprulethickness=1pt]
\setupTABLE  [1]   [first] [bottomrulethickness=2pt]

\stopsetups

\starttext

  \bTABLE [setups=tableborders]
    \bTR\bTD[nc=2] Lorem                               \eTD\eTR
    \bTR\bTD Ipsum  \eTD\bTD Dolor    \eTD\bTD Sit     \eTD\eTR
    \bTR\bTD aaaaaa \eTD\bTD bbbbbbbb \eTD\bTD ccccccc \eTD\eTR
    \bTR\bTD dddd   \eTD\bTD eeeeeeee \eTD\bTD fffff   \eTD\eTR
  \eTABLE

\stoptext


Marco