I just tried that and exp comp works by shifting the ISO and holding the aperture and shutter speeds.
Maybe try AEL on the Canon and then exp comp?
No, that doesn't work, I just tried it. I have seen discussion about this restriction several times and the people who like to use it all want it to be allowed, as it is allowed on Nikon. I have never used it, but I can certainly see how useful M + auto ISO would be in some situations, particularly if you can also use EC.
Sony doesn't allow M + auto ISO -- it doesn't on my A700 anyway. If you use M mode it does not allow you to select auto ISO.