I finally managed to contrive two simple examples, one that works  
here and one that doesn't.

The next code typeset with texexec --pdf with lines commented OUT  
does work:

\setupcolors[state=start]
\usetypescript[luc]\setupbodyfont[luc,10pt]
\startMPenvironment
	\usetypescript[luc]
	\setupbodyfont[luc,10pt]
\stopMPenvironment

\definecolor[gray][s=.825]
\definecolor[yellow][r=.625,g=.625]
\setupMPpage[offset=1pt,background=color,backgroundcolor=gray]

\startuseMPgraphic{standalone}
	draw (0,0)--(100,0)--(100,50)--(50,50)--(50,100)--(0,100)--(0,0);
	label (textext("cornerstone"), (10,5)) rotated 45 withcolor \MPcolor 
{yellow};
\stopuseMPgraphic

\starttext
%%%%%%%%%%\startMPpage
	\includeMPgraphic{standalone}
%%%%%%%%%%\stopMPpage
\stoptext

When I remove the %'s it does not work.
The first below is the wrong one, showing unknown, the second one  
correctly shows the word cornerstone.
Is the conclusion correct that it could be something with \startMPpage?

yours sincerely,
dr. H. van der Meer