I believe that it does use MOST, and if so that means no Phatbox integration is possible.
Yes, it is MOST-based. It might be possible to get a MOST-to-Alpine adapter and a new-Alpine-to-old-Alpine adapter, and connect the Phatbox / BMW adapter to this. However, that's an awful lot of adapters / converters and places for things to act up.
If the headunit interface firmware in the 805x ever gets figured out, it should be possible to emulate other types of changers, and then all you'd need would be a MOST-to-whatever adapter.