I'll talk with Jonathan tomorrow about this, but when you say:
"In particular I still only have a general sense of what is involved in the branch interposition model - to be specific, it's not clear what the transport requirements would be for negotiating the XID space; you mentioned alternate XID formats as well as bitmask-based solutions, but none of this is really specific."
I don't see how this will have any requirements on Remoting other than as a communication channel, i.e., the protocol should work just as well (conceptually) using HTTP, raw socket, IIOP or carrier pidgeon.