I don't know if you've ever got into podcasts, but "The Drunk and the Ugly" site has uploaded something that deals with drugs quite a bit (in Call of Cthulhu terms at least). It's called "Lover Before the Ice", and is available on their site.
One thing of particular interest is a drug (it's name escapes me right now) that induces powerful visions but only for about 30 seconds ... perfect for a RPG. They say it's much more intense than an orgasm while it's in effect, so might be exactly the sort of thing you're looking for.
It would (at least) avoid the problems of, say, dropping acid ... where you're going to be tripping for eight hours at a time. I honestly don't see Monty being interested in pot (way too mellow for the characters liking), so something like that would seem perfect (and you avoid the problems of having all those who partake being under penalties for a game day).
That's not to say there couldn't be some interesting side effects tho ... let your imagination run riot