Hi,
Not sure whether that's the best solution or not, but your example seems to work on my side if I replace \starttable with \starttabulate and \stoptable with \stoptabulate as follows:
_________________________________________________________________
\startuseMPgraphic{recBox}{w,h,sx,sy,lColor}
numeric u; u := 1cm;
draw unitsquare xyscaled (\MPvar{w}, \MPvar{h}) shifted (\MPvar{sx},\MPvar{sy}) withpen pencircle scaled 3pt withcolor \MPvar{lColor};
\stopuseMPgraphic
\defineoverlay
[kBox]
[\useMPgraphic{recBox}{w=2u,h=LineHeight+4pt, sx=.8u,sy=LineHeight-2pt,lColor=darkred}]
\starttext
\framed[frame=on,background=kBox]{
\starttabulate[|cw(.7cm)|cw(.7cm)|cw(1cm)|cw(1cm)|]
\HL
\NC \NS[2][c] q \NC\AR
\NC p\NC \NC 00 \NC 01\NC\AR
\HL
\NC 1 \NC 2 \NC 3 \NC 4\NC\AR
\NC 1 \NC 2 \NC 3 \NC 4\NC\AR
\NC 1 \NC 2 \NC 3 \NC 4\NC\AR
\NC 1 \NC 2 \NC 3 \NC 4\NC\AR
\NC 1 \NC 2 \NC 3 \NC 4\NC\AR
\HL
\stoptabulate
}
\stoptext
_________________________________________________________________
Best regards,
Florent