You'd also have to unload it in complete darkness: this is assuming what you'll do is open it, put it in, shoot, then take it out and develop... You most likely know that the cartridges are opened by a notch in the baseplate of the Ms and I presume by you saying the Rs aren't designed for them, that these cameras do not have such a notch. You would probably have to do some cutting down of the protrusions on the top of the cartridge to make it fit, such as the round knob and the flat spring.
Just playing with an IXMOO cartridge and the cameras around my computer, the Voigtlander Vitoret R will take the cartridge but with the opening pointing in the wrong direction and the Canon AE-1 Program will not accomodate it at all, let alone close with it inside.
I'd say you could make it work with modified cartridges but perhaps you'd be better exploring another option as this will not only require you to hack at nice out of production cartridges but restricts you to having to go into the dark each time you want to change film - a real pain out "in the field".
*i say cartridge, you say cassette*
*if I've addressed completely the wrong issue, please excuse my ignorance*
