It would be fascinating to have research into what is effectively manifesting the experience.

My (extremely limited) understanding is that humans are "hardwired" for pattern recognition (especially faces) -- could there be some sort of ROM equivalent of "this pattern is another human" that gets engaged by the chemical experience?

Maybe. It's certainly the case that there are areas of the human brain which are specifically associated with recognizing faces, like the fusiform face area - it's an intrinsic capability of the brain, not one that's learned. But I'm not certain that's sufficient to explain what's happening here. Increasing the activation of that area would most likely result in a person perceiving faces everywhere (e.g. an extreme form of pareidolia), not entire humanoid figures.