Take (amplified Axe-Fx output) and connect it to any reasonably heavy-duty full range driver, 5" or smaller. Now affix that speaker directly to the body of your electric guitar! Depending on the size of the driver, this could be on the lower bout, or down below the tailpiece, or even up on the headstock.
Obviously, if you’re playing a valuable pristine ax, you’ll want to put some careful thought into exactly how you attach this hardware. I’ve used duct tape, C-clamps, and reusable weather stripping, all with adequate results; admittedly, though, the best results I got were when I decided “to hell with cosmetics” and used wood screws to bolt the speaker onto the body of a cheap Japanese Les Paul copy.
The point of this exercise is that by re-energizing the guitar body with its own amplified output, you’ve effectively harnessed infinite sustain and controllable feedback. (Astute readers may recognize this as the theory behind products such as the Sustainiac, but I must tip my hat to Steve Holland and Seth Gussow for introducing me to this technique a decade before those commercial devices were available.) If you have a spare amp head that you can dedicate to this feedback circuit, you can also experiment with an equalizer in the signal path; this offers more control over which frequencies will feed back, allowing you to even out the response over the range of the guitar.
