Beringer has not published Xctl specification, but from my test it is just raw MIDI transfered throw the network pipe.
All parameters are in 0-127 range, the same for A&H.
I mean for what (N)RPN is good? Yes, it allows more parameters to be specified (CC is limited by 128x16), so some use NRPN other SysEx. Note that both are extremely slow in transfer (f.e. NRPN is 4 messages per parameter), when physical carrier is normal MIDI (~1ms pro message).
OSC has no limitations in this respect. Its design say nothing about syncing, but for controlling purpose that is normally not required (till someone try to use it for automated operations, there are such threads there).
The fastest approach is adopting OSCII-bot. Proper approach will be dedicated csurf (here proper value mapping can be implemented, to match particular DM FXes with particular VST FXes). But both is hard, long and not payed work... so no one has done it so far
