Specifically looking for spots to bluff raise OTR is a situation that is not going to come up often. You have to find a special kind of player and a special kind of board. For example, you CC w/ QJs in late after 3 others CC the UTG raiser. The UTG raiser is a barrel monkey. The flop comes 985 with one of your suit. UTG bets the flop and (magically) everyone folds. You call. The turn comes 2 of your suit. Barrel monkey fires again. You call. River comes the offsuit A. You should consider raising if he bets.

That's just an example. However, I think you would find many more opportunities to gain value (especially in loose games) by focusing on river opportunities to value bet, value raise, value calling (like calling when HU with not much because you're getting 10 - 1 or w/e), going for overcalls, raising to reduce overcalls, figuring out when YOU should overcall (IE: because the first caller will call w/ bottom pair), recognizing the pot is large and how that changes river play and inducing bluffs (especially verus aggro players at a HU river). Learning the intricacies of these plays and the player types that you need to be against and their tendencies will ultimately lead to a developed sense of when a river raise might be a profitable play.

I estimate a good spot to bluff raise the river will come up once in over 20 hours. I would guess that if you never make this play you might not miss it much. Notice that I'm talking about bluff RAISING the river, not bluffing the river... which you're going to have to do on occasion.