Don,
The Korg OS can do this.
Each user voice has full prgrammability of MIDI Bank/Program change. For each User voice you can select anything you want to be sent out to control a external sound module. Also, you can select internal/extenal or both for each voice. So you could have combi's as well.

I had asked Yamaha for that a long time ago..their answer was no, you don't need that as the T2 has everything you need built in. Home keyboard mentality for sre!