Hey Pickslide, get ready for a huge post!
It's funny - I see you heading down the same path I did not too long ago in trying to chase down the tone of the infamous Reverend Willie G. I phoned up Scott at axeandyoushallreceive.com and we had a great talk about pedals and BFG's tone. There's a lot of things going on there beyond pickups and pedals. Although a pedal like an Expandora with some good PAF's will get you in the ballpark there's a lot of mystery about the complete package. Scott was telling me there's a legend about Mark Knopfler hiring BFG to do the guitar part for Money For Nothing but when Gibbons when in the studio all he did was barrage him with questions about his sound which apparently caused him to leave. Another story about BFG's tone was from someone in Queen's of the Stone Age talking about how excited they were about BFG coming into the studio to see what gear he brought. Apparently he walked in with a Billy Bo guitar and some amp modeller, plugged into the board and just let it rip and it was just 'him'. I've got an M-Audio Black Box and it comes with a setting called 'Sizzle' that definitely has the early ZZ Top vibe, but that's the closest I've managed. Scott seems to think he might use a wah in a fixed position for some sounds, but I think the important think is a tube amp that's cranked up. His live sound is definitely different than what goes on in the studio - apparently he has been known to use a slew of Expandoras in a christmas tree arrangement :confused-smiley-010 . He talks a little bit about his sound in his coffee table book - Billy F Gibbons - Rock n Roll Gearhead - which is a fun read in itself, but there's a lot going on and he likes to generate a lot of mystery. One thing that makes a definite difference is that he uses 8's for strings ever since BB King apparently told him to stop struggling with heavy strings and to let the amp do the work. He also uses a mexican peso for a guitar pick - that probably has something to do with it, but to quote him and countless other guitarists the sound is really in the hands. I certainly with I had his!