What if you want to control per-note brightness instead? So it would control the filter frequency of a synth, for example?
That's where the MIDI Keyboard Mapper comes in!
It's available via the Rec menu at the top of the screen. The 'Convert incoming controls' option is what you're looking for.

OK, this picture is from the desktop version, and it doesn't show converting Per Note Aftertouch to Per Note Brightness, but you get the point

FTR: Per-note brightness works with:
- MultitrackStudio Instruments (the synths)
- AU plugins that support MPE
- AU plugins that support MIDI 2.0 (per-note controllers). There aren't any yet AFAIK.
Giel Bremmers