Great to have posts on this subject.
I can think of no reason not to bottom because of BPH. Go ahead and enjoy yourself!
There is plenty of space in the rectum behind your prostate, so I would suggest that you are just guided by what your pain receptors tell you: stop if it hurts.
My husband and I have ended up enjoying all sorts of things we never did prior to my post-prostatectomy-ED. I used to be almost exclusively active and the initiator, but he is now more dominant than he was, and it has been lovely!
MacMillan are getting interested in LGBT people's issues with health services, if you share the concerns others mention here.
I have found Metro Walnut support group in Greenwich very useful, and the is a Manchester-based PCa group for men who have sex with men.