Yes, it will interface with the RM4XS. Currently, the buttons aren't assignable, but that's because they're not currently implemented in v1.1. Stalk controls won't work in v1.03 - just basic tuner support.

There's also (currently; not sure if it's made it to production) an ID switch which allows you to set an ID for the tuner module. This means, as the tuner module is permanently fitted into the car, that in future software we can have car-specific EQs saved (and maybe it'll pick up from a different bookmark depending which car you're in).

The port, with some inverters and a MAX232 would be suitable as another serial, yes. Put some current limiting resistors inline with it, as the early mk2s didn't have these onboard.

Hugo