Right, maybe this should've been a new thread. I was just thinking about sustain in general.
I found an inefficient workaround -- the cpwitz/midi/arpeggiator "hold" function does exactly what I wanted. Keys that are being played stay frozen when hold is engaged, and my envelopes take over when hold is disengaged.
My patch has 8 voice polyphony, and it seems to be handling it with no problems.
Do you know of a way to do this without the arpeggiator? I've experimented with some "note hold" objects to no avail. Currently at 95% DSP -- it would be nice to slim it down.