In case this might be useful information for someone, I had a cysto to investigate this constant and mild burning in the posterior urethra six months out from my RP. The doctor told me that strictures (narrowing of the urethra) normally present themselves around the six-month mark. However, the scope showed good healing, no stricture, and no other pathology. Labs indicate that I don't have any infection.
Doc's conclusion: the mild burning is part of the post-op healing and adjustment going on in the area of my RP. I'm willing to go with that since no pathology has turned up in other tests. The burning is mild and can easily be lived with.