Well I hit on a great idea in one of the ntc threads, and tried it last night.
I set up my guy, who has been locked up with no release for two weeks. I had him kneel on the floor naked, with a small mirror on the floor beneath him. I told him to masturbate to the edge and stay there, and let me know when he was ready. (After two weeks, this part did not take long!)
I then set up a kitchen timer and told him I would set it for 5 minutes. At the end of 5 minutes, there better be pre-cum on the mirror. When the timer goes off, he will have to lick the mirror clean, and start over.
This forces him to be close enough to the edge to leak pre-cum steadily.