Try adding prologues := 2; or prologues := 3; in your mp file.
Good thought, which I tried, though neither setting resolved the problem.
Any particular reason that you are compiling metapost files using mp and then including them in ConTeXt rather than directly using any one of the inbuilt ConTeXt environments for handling mp (\startMPcode, \useMPgraphic, \processMPbuffer, etc.)
Mostly conservatism. I cannot quite figure out the canonical replacement for btex ... etex (textext, sometext?), and then I get confused by the many possible environments. What do you recommend as the best btex...etex replacement and the best environment for replacing a standalong metapost figure (that I would otherwise just include with \externalfigure)? -Sanjoy