[way off topic]
TwoHands: Essentially, yes... that's what a Sustainer is. Pickups are essentially electromagnets in reverse... move a metal string through a magnetic field and the resulting fluctuations produce electrical current in the coils. Sustainers work on the reverse principle... apply enough current (and oscillating in the right way) and the resulting electrical field can move the strings. The eBow works kind of this way, though it doesn't rely on the guitar's built-in pickups... it's just a string exciter.
A Fernandes or Sustainiac sustainer uses a small amplifier that gets installed onboard, which takes the signal from the bridge pickup, amplifies it and pumps it out through the neck pickup/transducer, further exciting the strings. (I believe Michael Brook's "infinite guitar" system works the same way, only i think he uses an external amplifier. He's very secretive about it, and only really says that it's different from the way the Fernandes/Sustainiac sustainers work.) The key is getting the amplifier and transducer to respond well and pump out the right kind of signal and magnetic field, which is what Sustainiac has been developing for longer than anyone else.
The only other time i've seen a guitar with an input jack is on Jerry Garcia's custom "tiger" guitar, which had an effects loop that he could control right from the guitar. Signal goes out to effects, comes back into the guitar, then out again to the amp. Nutty guy.