FWIW (other people have more experience than I do) --
An interesting idea, but here's one problem:
. . . The most sensible way to play drums, with MIDI, is to have a "drum set voice" on the
. . . target instrument.
A "drum set voice" is a voice which has one drum sound _on each key_. So C4 might be a bass drum open, C# = bass drum closed, D = low tom, d# = high tom, etc etc etc.
That's the way that "arranger pianos" get their rhythm tracks. It lets you use lots of different sounds, without using a lot of multi-timbrality or polyphony.
I don't think there's a _standard_ for that "percussion sound to MIDI pitch" mapping.
My PX-350 has 14 different drum sets, in its "General MIDI" voices, numbered 129-142. I _think_ that puts them outside the "universally supported" General MIDI range.
So you'd need a "drum set library mapping" (or several of them) for each instrument that you wanted the "MIDI drummer" to work with. Not impossible, just a hurdle.
If a DP has a "Line In" jack, it's far easier to use any of a huge number of "robot drummers" with audio output _and dozens of standard beats built into them_.
Something like the Digitech SDRUM (very new), or an Alesis SR18 (old, now), is a remarkable piece of technology. It's not a _real_ drummer, but to many people, an NU1X isn't a _real_ piano.<g>