Hi Colin,
"While it may be "technically" possible, the images will be out of focus."
I don't believe that is correct information. From my understanding, there is no difference in the part number for the two pressure plates. They both press the film up against the dimensionally same film gate (film plane), 120 or 220. The film gate (and film plane) for both the 120 and 220 backs are at the same same position, as they should be.
The differences in the two backs are in the gears, which account for frame spacing and for frame counting. You can convert an A12 back to an A24 in fact:
http://home.earthlink.net/~blackbird711/manuals.html
"This is the same reason one cannot(again, technically one can)use 220 film in a 120 (A12)back."
Again, the pressure plates etc. are exactly the same for the A12 and A24, so there is no reason, except frame spacing (which is different between the 12 and 24 backs), and number of images/end of roll, that you should not be able to use 220 film in an A12 back.
"Cameras that allow for both types of film within the same holder, have a rotating pressure plate for the different film types."
Not entirely true. The Rolle 2.8F 12/24 does not change the pressure plate, the only difference is in the frame counter.
There is a difference, as you state, in the thickness, but whether you have to change the pressure plate setting or not is really design dependant, and as such, some cameras need it, and some don't.
Regards,
Austin