Great question, one could use existing known chemicals as a starting point. There could be a potential to use fMRI readings on a model organism in realtime to generate data.
Compelling. I wonder what else this could be applied to in addition to psychedelics? Anti-anxiety and other sensory affecting drugs?
If you wanna get Black Mirror-esque, perhaps a Soma-like medication from Brave New World (essentially pacifies/zombifies you by creating endless bliss) could be made. Or the "bliss" drug episode of Doctor Who.