If you watch the new tutorial videos for OS5 you will hear Ben Crosland specifically saying that Access do not allow it since envelopes are per note, and the FX are per part. But I ask, why not allow it exclusively in all of the 4 monophonic modes?
Anyway, I think this can be worked around using the modulation matrix. Control the desired FX value in a modulation slot with a constant as a source, and modulate the modulation amount of that slot from the envelopes (from another slot). Add more FX slots like this one to increase the amount of the change. Didn't try it yet myself.