Yes, but reports here are that BA frequently won't allow pax who aren't flying on BA into their lounges. I think the "out" they have must be that Lounge access may not be available due to capacity constraints during peak travel periods... from www.aa.com/oneworld, although i suppose you could raise a stink and ask them to prove that the lounge is at capacity until they relent and let you in...

I will be flying IB (marketed and operated), and don't want to risk getting shut out (shower is most important), so I'll just take the "sure thing" shower at the AA Arrivals lounge, then head over to the BA F lounge, and if they let me in they do, if not, no huge deal.