If there's already a way to achieve this random static value per voice (I might have overlooked something), then I'd love to hear about it!
Sort of, as I recall from my Mopho days. A random LFO assigned to frequency (or whatever), at the slowest possible speed, with key sync on, and you'll have a random value with each gate for a good long time (as long as you're not holding notes for more than like thirty seconds).
This technique also works for the Evolver (although the way to turn on key sync is silly and hidden [using the the Amount control to go all the way to the highest amount, and past it]), where I use it sometimes. On the Evolver, the slowest LFO period is a minute, so it might as well be a static value.
Apparently, this technique does not work for the Pro 2/Prophet 12 architecture, because a key-synced random LFO always starts on a value of zero for those synths, and (from what I'm told) changing the LFO phase doesn't help.