I really like Sonny's playing though I have never seen him live.
I saw a guy do the "fretting behind the
slide thing" about twenty years ago. I think his name is Scott Colby. He was opening for experimental guitarists Fred Frith and Henry Kaiser. I remember reading something by Kaiser where he said to the self taught Colby about fretting behind the slide, "that's really wierd!" Kaiser, whom I believe has transcribed many of Beefheart's pieces, amongst amny other things, calling something "wierd" will definitely get my attention.
IIRC, Colby was doing a lot of the minor half step thing, but also more complex things like a seventh (a whole step down) and some two figer chords. I've forgotten about him, but I should look up his stuff since I remember being impressed by him.
I try to do this occasionally but it takes a very light touch with the slide, or nastyness happens. Like everything else with slide, it takes a firm but gentle and light touch and a huge ammount of practice and perseverance to get anything useful out of it.