I built a small GPT -4 API using Nostr with a similar concept in mind. Its working live at geppetto.lat , and you can find the repo at

GitHub

GitHub - 42Pupusas/geppeto: A GPT-3 access point through Nostr, powered by lightning

A GPT-3 access point through Nostr, powered by lightning - 42Pupusas/geppeto

I'd love to hear your thoughts on the token system I designed, the bot posts Nostr notes (almost like a proof of purchase) in response to payment, and deletes a token every time a user sends in a prompt.