I dug up some more info on the Infinite Guitar:

Technical description of Guitar Wiring and Black Box

The guitar has an Electromagnetic Transducer in the neck position. The signal from the bridge pickup (Seymour Duncan) is sent through a black box that has a circuit board inside. It serves as an Equalizer that changes the Harmonic structure of the tone of the note(s) played. It boosts and cuts frequencies to make different overtones more or less predominant in the tone of the sustained string. Then the signal goes from the Black Box Back into the guitar and to the Bridge transducer pickup. So basically the signal from one pickup is sent into another pickup making the signal feed back into itself. This creates a feedback loop and the string is constantly vibrating once the transducer begins to move the string by itself by electronically exciting it. (Pulling and letting go of the metal string rapidly and repeatedly. So fast that you can't hear the beats of the pulse.)

-anonymous recording engineer and producer, December 1997

Alan Hoover, of Maniac Music, on the Stealth Plus (Fernandes Sustainer & pickup)

We first made our acoustic type sustainer in 1986 (the Sustainiac Model T, soon followed by the Model B). Then, we heard about the Brook Infinite Guitar when U2 played in Indianapolis in late 1986, as I remember. Edge's guitar tech called us, and allowed us to play a few notes on the instrument after the afternoon sound check. I thought that seemed like a neat way to make sustain, so we designed our own version of such a sustainer after looking at the I.G.

The Infinite Guitar used a regular Duncan stack pickup for a driver. Since this is a high impedance device, it requires around 100 volts of drive signal to produce adequate magnetic drive into the strings. This seemed kind of crazy to me, so shortly after that the Sustainiac GA-1 was born. We made a low-impedance driver so that the sustainer would run efficiently on batteries. The driver could be used as a pickup by attaching a transformer or amplifier to its output in order to increase the voltage output.

The "G" is for Gary Osborne, my partner in Maniac Music. This was followed soon after by the GA-2. We subsequently filed and were granted several patents on our refinements that allowed the magnetic sustainer to be used as a practical, manufacturable device. We never attempted to patent any of the basic principles, such as a pickup being used in reverse to drive the strings, because we always felt that that credit belonged to Michael Brook.

Curiously, Michael (whom I met and talked to at length in 1990) never followed up with his British patent that he filed sometime around the 1985 timeframe. Typically, patent offices reject first applications over technicalities. It is up to the inventor to persist and argue his/her case. Michael gave up and didn't argue his case, so he never got a patent. He told me in conversation that he was very busy, and also really didn't know that you could present an argument and maybe get your patent.

Then, Floyd Rose et al got a U.S. patent on a magnetic sustainer driver in 1990. Curiously, they claim never to have heard of the Brook device prior to making their sustainer. I know of no facts to contradict this. We have a patent cross-licensing agreement with them.

The new Stealth Plus sustainers are proving to be successful for the retrofit of existing instruments. They have the advantage that for most installations, no permanent change needs to be made to the instrument, so that a vintage instrument can be restored back to its original condition should it be desired. Furthermore, the bilateral driver (our U.S. Patent number 5,932,827) allows almost complete cancellation of the driver's radiated magnetic field that is sensed by the pickup. Consequently, for the first time, single-coil pickups can now be used to supply the input signal to the sustainer.

I'd like to build one. A built-in feedback generator. It would be nifty to cobble one together out of parts and secure it in some ugly way to one of my guitars. If I was using it just to generate atari teenage riot-type screeching, form ought to follow function.