I haven't been able to bring myself to raise the strings high enough to fret behind the slide. This is giving me the urge to try with my Danelectro -- it has a screw-in metal nut which can be easily shimmed. A lot of that behind the slide fingering can be mimicked by just moving the slide. But this makes the movement of the slide rather frantic, mutes the standing chord, and is less elegant than Sonny's style. To me the main advantage would be the immediate availability of minor chords, by fretting behind the major 3rd, which otherwise must be creatively faked in the usual D [E] or G [A] major tunings.
Technique aside, these videos were great. Thanks!