I've used the Black Rapid double strap with a 300 2.8 No Problem, and also a 100-400 on the other side, worked fine, Still had a FF camera with a 24.-105 around my neck....
I don't think I'd hang a 500 off a BR strap...... Thats getting up there in weight..... On the 500 400, 800, I carry it with a real Canon Strap that comes with the lens and leave it attached to the lens, works really well....
Oh yeah, 500 on trip pod over shoulder... Well, I carry the 500 on a tripod, I shorten legs and carry it vertically with the lens resting on my shoulder and holding the tripod. Don't trust any mount enough to have it hanging behind me, while the tripod is horizontal... Also put lens strap on me someway to be sure it doesn't get dumped. I've dropped a few here and there, never had one hurt yet....
Best wishes
Mark H
Too Much Camera Stuff......