Oh, man. My excitement about the CMUCL/MPS integration seems to have been premature :-)
Paraphrases from the early part of the MPS tutorial:
"We didn't actually get too far with the actual implementation of MPS and CMUCL, since we were unable to boostrap CMUCL if we made any (even tiny modifications)." (But apparently they have all the design issues solved).
"Unfortunately Dave Jones who's been doing the work on this is ill and thus not at the conference."
"Used CMUCL rather than SBCL since Carl Shapiro had earlier expressed interest in integrating MPS and CMUCL. No particular reason besides that." (In answer to my question about why they didn't try SBCL if bootstrapping was a problem).