Midi is a standard that all electronic musical instruments understand, if you design something specific for arrangers then you totally defeat the object of a universal multi keyboard communication protocol.
GM3 when it is sorted will most likely have it built in anyway, (As a universal standard not just for arrangers) so that any electronic instrument that wants to use it, can. (The manufacture will decide whether they wish to add it to their Midi implementation or not)
My guess is that GM3 will be a common protocol that can be used via Midi sockets or USB. (We will just have to wait and see on this though)

Bill