There was a member over on TDPRI who wound up buying a gutted MCI Guitarorgan. They didn't know what it was; simply an ES-355 copy with each fret cut into six sections and all sorts of square holes cut out of the body. Apparently MCI started off with a good Japanese copy and worked from there.
The demos I've seen on youtube tend to have the phenomenon whereby it sounds like all the notes are on at once, but quietly in the background. I think this is the same sort of 'leak' you get with transistor organs, like Farfisas, so I imagine the technology is much the same.
The beast I want to know more about is the WEM Project 4/Fifth Man guitar. Prior to any guitarorgans, or at best parallel with the Vox one, WEM came up with some odd sustainer technology that they built into one of their models. The instruments themselves have chunky magnets either side of the strings at the heel of the neck, and a crude hexaphonic pickup arrangement in the bridge. I have no idea how any of that worked, and I cannot find any resources online beyond anecdotal evidence.
Apparently the magnets promote sustain, but I don't see how that would work! If anything I can see two chunky magnets introducing wolf tones and sustain issues if anything.