Maybe I'm wrong here, but is there such a thing as a MegaExo frame? The MegaExo bottom brackets that I've encountered have come in two versions: English (68mm) and Italian (70mm) threaded. Shimano stuff like the 5700 BB will work on an English or Italian threaded frame no problem.
As for mixing Shimano cranks and MegaExo BBs, there can be compatibility issues, so I wouldn't recommend it, especially given how cheap it is to get the right BB.
Last edited by dr_lha; 12-10-14 at 01:09 PM.