I don't know too much about Bob other than he built some pretty cool choppers and was once a partner at U.S. Motorcycles in Lynwood, Ca. I do know, that like Dick Allen, Bob tended to lean towards progressive performance based choppers with features such as mag wheels, belt drives, collector exhaust, braided lines, custom forks, tanks, and frames.
I ran across Denny Cukr's bike in an old Street Chopper. Amongst other things, it has a Bob Hall semi-drop seat frame. It's one of the only aftermarket frames I ever liked. I'd been meaning to post it, then found the scan below on the LoserRules blog from the same article.
When I get the chance, I'll dig up some other Bob Hall bikes.