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That's too bad. I thought there were at least some VIs that will respond to program change. In fact, there must be because I'm pretty sure Apple's "built-in" DSL Music Devices does. So it is technically possible I think... just not supported by many developers.
There is a script in Mach V 3 which allows it to receive MIDI program changes from an external device and switch between different sounds. Program change #1 activates the patch you have set on MIDI channel 1. Program change #2 activates the patch you have set on MIDI channel 2, and so on. I believe this was a new feature in DP 7. I use it for live performance. I assume it could also receive program changes from a DP MIDI track. Would this work for you?
Program change messages make sense when you're changing patches on a synth or changing sounds burned into a ROM, where the change is close to instant. But changing the sound in Mach Five or any thing that has to load up samples, is going to result in an unpredictable delay, and interruption in playback, you probably don't want. If the samples are all in RAM already, I suppose it could be useful, but then you might as well just put the sounds in different channel/VI combinations and change playback to a different track in DP.
The script in MachFive: if that activates a program already loaded, rather than change the program loaded into a slot, it's not really the same thing.