Robert seemed to do the most interesting variations in the first 4 bars of his 12-bar progression, perhaps because he was most comfortable playing off of A chord fingerings. Here are 3 different ways that he played the first 4 bars of his 12-bar progression in the song "32-20 Blues," which is one of my favorites. Notice that the first example switches between 2 A fingerings for the first 3 bars, whereas the 2nd and 3rd examples go from A7 to G#7 back to A7. In all of the examples, the 4th bar features a simple 3-note run from A to G with slight variations. In the actual song, Robert played lots of variations on that little 3-note run, sometimes extending it to 2 measures and fitting lots more notes into it. Try experimenting with it -- you can play pretty much any note out of an A7 arpeggio or A blues scale and it will sound OK.

Oh yeah, by the way, the 3rd example doesn't really utilize the thumb bass, but it sounds real cool when Robert does it...