The speculation about "foreign" backs on different cameras is largely nonsense. Unless you're one of those people who is simply fascinated by machinery (as opposed to photography) why would you spend, say, $30,000 on a back that may or may not work well with a particular camera, that may or may not be supported in the future, whose firmware upgrades that may work well with the native camera might not work at all with your combination? Could anyone honestly say that they will see startling differences in the print if they were able to use, say, a Phase back on a Hasselblad rather than the Hassy back? And assuming that such a thing could even be done, there would be so few sales for Phase, you have to ask how much effort Phase would put into supporting such a project five years in the future. My estimate: none.
The hard fact is, I believe, that we will have three systems (or four, depending on how you count the Hy6). They will produce very similar IQ, with few differences except possibly in extreme blow-ups; those slight differences may be reflected in price (as between Mamiya and Hassy, for example.) So we wind up back in the film era: do you want to pay more for a Hassy, and perhaps get a slightly better lens and more prestige, or less for a Mamiya, and get everything you need for the high-quality magazine repro?