Thats a great idea... i was doing that with an modular OSC and external ADSR :-)... and it sound fun... also when adding a long attack on the OSC with longer release (question/answer game)
seems like it coul work internally, though i think you have to:
a) set osc levels to 0
b) add the ADSR and...
c) make sure, that the hard wired VCA ADSR has bigger/longer sustain and release...
will try that tonight :-)