I can't think of any - A/D input stimulus is setup to modulate notes that are already triggered by something else (or, in other applications, change tempo).

Rather than trigger a note - you can have a note sounding constantly but only have the volume at an audible level when the snare hits. So you gate the output (off when snare not hit, on when hit). This will sound like triggering notes - but really the note/sound will already be triggered by something else and only allowed to be heard when the snare hits.

This works for notes that have no special attack vs. sustain sound. Because you'd only be opening the gate during the sustain part of the note. And I would use FM-X for the note since FM-X can easily "drone forever" from a single trigger.

A somewhat related follow-up to this. Is there a way to trigger notes using MIDI input (not A/D?)

My drummer has drum pads which send midi note on/offs -- could I use those to trigger a held chord in the MODX for note-ons, or to trigger the amplitude or filter envelope as you suggest above?

MIDI notes from an external source would trigger their own notes or translated through an ARP. Sounds like you want to hold notes and have an external MIDI device provide the note-one messages for your held piano keys. There's not a solution for that I am aware of.

Yes - these MIDI notes can trigger filters and amplitude. You could take these MIDI notes and have them trigger a dedicated Part that you set the output to OFF. This way your keyboard wouldn't "play" the notes your drummer is sending. What would happen is that internally these notes are producing an envelope using envelope follower and the output of the follower would be your modulation source.

There's also effects that use modulation sources that may be an option too depending on what you're trying to do.