I've used an extreme range of drugs over 14 years and I'm sorry to say none have been conducive to hypnagogia for me. Using diphenhydramine puts me straight to sleep (50 mg) with no layovers in hypnagogia land. Are you referring to the deliriant effects of DPH as being hypnagogia like? I've never taken DPH recreationally, but the descriptions don't sound like hypnagogia, which I am very familiar with because I've practiced entering the state off and on for a long time. The best way for me to get into it is to wake up early, eat breakfast and dick around for an hour, then go back to bed. While falling asleep I try to listen to the environment around me (basically just to keep conscious attention as I near sleep). I start hearing brief phrases in different voices, and not long after get imagery and sensations (lucid dreams are much more likely to occur if I've been trying to enter the hypnagogic state but go too far and fall asleep).
Calea zacatechichi (the "dream herb) might help, though I can't say from experience. Dissociative induced "mind movies" are probably the most similar to hypnagogia I know of from drugs. Sensory deprivation is the closest I've gotten to it without actually starting to fall asleep (but that's really expensive). Sleep deprivation from meth (only tried once) was also very similar. I tried to go to sleep 48 hours after first dosing and kept getting very realistic visions of scenes from the rave I had been at. But staying up like that is obviously unhealthy. On the whole the hypnagogic state strikes me as too fragile and fleeting to extend naturally from recreational drug experiences, as the sensations of recreational drugs demand too much of your attention.
EDIT: When I've taken caffeine and still felt exhausted despite it and so took a nap to refresh I've gotten some really impressive hypnagogic experiences. It seems to help hold me back from going into deeper sleep even when it doesn't supply the wakefulness I take it for, and that's conducive to hypnagogia.
