Howard, at the very least, Leaf backs on Mamiya bodies is a common combination.
In general, I think it was a good move by Phase One to do this. If Mamiya had tanked, Phase would have been in the position of fitting only no-longer-produced bodies, which despite the strength of the Phase backs probably would have proven impossible to survive.
The Mamiya body is at this point probably the least advanced body on the market, with certain known limitations, and needs dedicated backing to move the platform forward.
However, if body-building (haha) is not a profitable venture, as seems to be indicated by all the camera manufacturers with trouble, then I don't see the continued investment in open standards and publishing specs and protocols as being an attractive option for Phase. Essentially, once the platform is in good shape again (body brought up to date, leaf shutter lenses released, T/S lenses fixed), Phase will face the same predicament faced earlier by Hasselblad, namely that customers will invest in a Phase body, but go with a different back, thus cutting out the most profitable segment of Phase's income. Sinar has already made minor steps in the same direction with the eSprit 65 back.
Being more or less the last major open player on the market will not be sustainable, IMO. What we will likely see is a strategy whereby the new body (or perhaps an upcoming one) will be locked to Phase and Mamiya backs (or Mamiya backs might be discontinued; they are not as good as the rest anyway, and with Phase in-house...), and the AFDIII (or another "previous" model) will be left in the lineup as the "compatible" body, just like Hasselblad did with the H2. Then with time, that option too will be closed.
This was all quite predictable when Hasselblad went closed. The open ecosystem only works when every major player participates. By going closed, Hasselblad has essentially forced the entire MF segment to go closed, for exactly the same reasons: building cameras is not profitable, building backs is (still).
Sadly, this development really puts pressure on both Leaf and Sinar, who would end up with no modern 645 camera in their lineup. Perhaps something could change there. Or, perhaps Dalsa develops a large square sensor to give the Hy6 a little wind in the sails. That would be very interesting!