I've been playing behind the
slide for years.
Its not that hard.
In open tunings you can get minor chords, m7ths, dom7ths,dim, M7ths, and other extensions behind the slide, theres lots if you can stretch a bit.
Also these can be used as arpeggio's for tasty licks and melodies.
Pull off and hammer licks to open strings (among others) work real well behind the slide too, as well as incororating the slide, with fingering for box scale licks.
Again, its not that hard, you just have to use your noodle.
As Barry pointed out, the guitar must have a high enough action for this to work,(but not too high).
I learned a lot from watching Ry Cooder, many moons ago.
And I've seen ol Kirky boy rip some nasty pull off's behind the slide too.
Makes slidin heaps of fun!
Get Behind!