I did some small examples, motivated by the article of Wesseling in  
NTG's "Voorjaar 2005" MAPS.
Latest context and fonts installed (2005-12-19).
I do not understand the results.

(call) texexec --mptex test-1.mp:

beginfig(1);
	draw (0,0)--(100,0)--(100,50)--(50,50)--(50,100)--(0,100)--(0,0);
	label.urt(btex \ss\tfxx coorner etex rotated 45, (50,50)) withcolor  
blue;
endfig;
end.

Why are the o's colliding?