Indeed, using the note number as a modulation source is not supported currently. We consider making more parameters "Note Edit"-able in the future, would that be a good solution for your purpose?

For what I'm trying to do right now, yes - the particular patch I was working on needs to shorten all the envelope times with increasing pitch, to keep the length of time air is supplied in a constant ratio with the response time of the pipe.

In general, though, I think including note number as a modulation source is probably a cleaner way to solve more problems for more users. In the process of refining a patch for performance use, slight modulations by note number can be useful across essentially every parameter on the entire synthesis engine. (For an example, in a patch I put together or find as a preset on a more traditional software synth, I would fully expect anywhere from a fifth to a half of the modulation links total to have note number as their source!) Besides, if your target user is mainly familiar with synthesizers, note number as a modulation source is the standard way to accomplish this kind of voicing across the keyboard; it's what they'll expect. On the other hand, if you're instead going for more organists who are interested in synthesis, I can very much see the reason to do things the way you are suggesting here!

Indeed, using the note number as a modulation source is not supported currently. We consider making more parameters "Note Edit"-able in the future, would that be a good solution for your purpose?

As tscheevel stated, "keytrack" is really the correct term for modulation by note number.  It should be capable of working in negative and positive mod values with mod level being a parameter - just as you have mod levels for all the other modulation sources.  So for example, assign keytrack to modulate volume and you can either get louder notes or quieter notes when you play higher up the keyboard.   It can be useful for a lot of parameter modulation.  This is one of the things I suggested as a beta tester.

Have any of you been able to find a way to use note number / keytrack as a modulation source in Airteq?

For anyone searching for the answer to this same question, but are only interested in setting the destination as the volume of a stop, this is easily available by right-clicking on the Volume knob of that stop, then clicking on the "Note Edit" button in the box that comes up.