The digital pick-ups CAN be used on acoustic instruments. I have seen a nice Martin with a digital pick-up - seemed to me a shame, actually; lots of cutting etc - but it worked. I don't know why you'd do it though. If I use a digital guitar I can play banjo, any guitar tuning I can dream, harmonica, bouzouki, mando, otave mando - etc. And no acoustic intrument required. I can do it all on my Strat!
But if you want some persnal experience with the technology, send an Email to Mudcatter, Grey Rooster - firstname.lastname@example.org
I'm sure he can tell you all about it. He's a very talented acoustic guitarist and a bit of a gear head.