Its an interesting question, and one that can work the imagination. I'd like to try and help sort out the answers from above, by grouping them:
- backs that have interchangeable mounts, by the consumer: Hassy CF backs (was Imacon) with I-adapter, and Sinar. These may require a falsh synch cord to fire them, which is not a problem, but isn't the most integral of answers. The mount swap isn't terribly cheap, but it is effective if you have many platforms to work with.
- camera adapters - mounting plates: the industry has made some adapters, where a mount made for one camera can go on another. These mostly serve the viewcamera market, but can be useful.
- mount changing by manufacturer - Phase for example will change the mount, but you have to send it back to them. Good for if you ever seriously change platforms, but not for everyday.
- some mounts are more ubiquitous (common) than others, and thus one can "cobble" a flexible system from combining several of the above: for example, a Hassy V mount, or a Mamiya mount, or even a Contax mount might be usable on those cameras, Alpa, Silvestri, or view cameras, thus allowing you to piece a more flexible system together. But you should scope that out before hand, as its not like everyone works off the same game card. in general, tho, some camera makers (like Alpa) make a range of different mounts.
- there are some dead ends that are still useful: I have a phase back in a Rollei mount (Rollei made this just for their cameras). The back and mount won't go on anything else, but I can move it to another Rollei - and I think to an X-act view camera (also by Rollei). Even the Leaf backs for Hy6 (surprisingly) can go on Alpas, for example. Its a bit weird to see the little nuances of this. Mounts for V Hassy can be used on all the V system... which is large. It used to be with the Hassy H1/H2 you could bring your own back to the party, but they gave that up (more on that below).
There is always the desire to have one back switching between different platforms. It seems to make sense, and thus the older CF Hassy and the Sinar backs are designed for this. There is no perfect solution though to this problem. Some will argue that back alignment is more critical than flexibility and the backs need to be factory aligned to the camera (thus not the flexible changeable mount system). Sinar addresses this with vey thin alum foil strips you fiddle with to get right. Alpa now has a shim system as well.
Another reason is for electronic protocols - sometimes EXIF data is not shared when changing platforms, if that's important. That is probably the reason Hassy closed their system with the H3 (along with other issues), for just their own backs.
In reality, the image of a common back (like a Hassy A12) swapping all around is compelling, but these digital backs are not quite used that way. They tend to prefer a stable home and mount.
Some do swap them around with success. Get either a common mount type, and pick your platforms carefully, and you can meet this goal. The other way is to pick a system with broad flexibility and be happy developing within that structure.
Hope this helps.
Geoff
