Canon could release a new mount / lens system for the MF body and an adaptor for EF lenses possibly in a cropped mode.
However, we'll have four mount - EFs, EF, M and MF. That's a lot of investment in production lines but if Canon can pull off MF it will certainly put the cat amongst the pigeons i.e. Nikon & Sony
Canon can do what they want. The technology exists.
The question is - does the market exist?
What would Canon bring to the MF digital market that is not there already, and would the resultant perceived value support pricing that would make the line profitable for Canon?
Folks get excited over the technical aspects of things, but one does have to remember that Canon is a money making concern, and unless the product can be supported by the market... its a no go.
I for one cannot see an "EF in crop mode" as part of any MF camera. If you can afford the MF camera in the first place, and have the EF lenses to use in "crop mode", you almost certainly have a better performing camera body that accepts those lenses.
Folks who "need" a large sensor, high resolution imaging system do not need Canon's forte (so far) of technical experience. Those folks wont be shooting fast moving, high FPS, hard to focus types of images with MF equipment.
I can see some marketing hype of "near MF resolution" from an EOS camera body, which reminds me of the "near 35mm resolution" claimed from the Pentax 110 system so many years ago (when I sold a ton of 'em behind the counter!).