This has happened to me twice, both in spas attached to upscale hotels in India. Both times, since it was clear that the massage made me hard, the masseuse volunteered to toss me off at the end of the massage. I said yes, and gave them a nice tip - which I think was the goal from their point of view.
If you try to "negotiate" something like this at the beginning, you run the risk making the therapist uncomfortable (as in the recent alleged Al Gore incident) and you are essentially propositioning someone for prostitution - which is probably illegal.
Probably the best thing would be to go someplace where others have had the same experience you want. I wonder if there is an internet site that give clues?