Thanks for the feedback. Unfortunately, I can't think of any easy way of scaling as you suggest. The note length isn't determined by the envelope, but rather by the feedback decay factor. The filters seem to be a factor as well, ie. for notes in the 50s, the low pass drastically changes the duration of the note. So you'd have to write a formula that balances pitch, feedback and filter frequency, which sounds too complicated for my poor brain.
This is not a very consistent synthesizer--the charachter of the sound changes, sometimes drastically, from one pitch to another, and in some cases the harmonic modulation even changes the pitch completely (try midi-note 41, FM harmonic 6, FM amount 1.04, LOP ~ 7000!) On the other hand, I'm quite enjoying its unpradictable behaviour, especially considering how simple the build is. I think I'm going to replace the MIDI note selector with a line input for pitch, as it's probably better suited for sweeping sounds rather than melodies.