Keep in mind that the the cocking lever does two things, it transports the film and cocks the shutter. If you modified the film transport to stop at a shorter length of film, you would end with a half cocked shutter. So this is nothing that can be done without a change of the gears, so cannot be done by us tinkerers, I believe.
The most sensible suggestion that comes to my mind is buy a TLR and forget about modifying a 35mm camera with all its inherent limitiations of resolution. A 6x6 negative would be getting close to 360% larger than 35mm and packing a substantial amount more detail in your photos.
A few Canon SLRs have IR/LED sprocket counters linked to drive, shutter, mirror and computers. I wouldn't dream at all of tinkering with these. Back to the suggestion of a basic 6x6 to play with.
Yes I agree with simply just buying a 6x6 camera. I am wondering if the new advanced model like the Nikon F5, F6 can be done by simply changing the firmware? These cameras have separate shutter cocking motor and film advance motor.
i'd Just mask the finder and the film gate of a 35mm camera.
If you want a real project, try converting a (the only) 126 Instamatic SLR to use 35mm film. That way you will have the correct lenses for the format.