This is a great question. I am super keen to do a 650b conversion on my frame, should make it so much faster over the rough stuff but still be just as much fun as my 26" wheels.
Only problem is, as you say, frame clearance. One option I was considering was tracking down some 1.8" 650b tyres so that I could do the conversion but still not affect my frame clearance. Has anyone got any suggestions for nice grippy, big volume 1.8" 650b tyres ?