Very nice idea! This enables a host of nice stuff for multiboxers, like showing every characters XP or Faction on their respective party frames. Very nice indeed, going to give it a try right away .

Code:
/rpc <alt> UnitXP player
/rpc <alt> UnitMaxXP player
To retrieve the amount of XP a character respectively has and needs in total to ding.

Edit: Any way you can let remote clients register for events?