I think Nostr-rs-relay and some others have a gRPC hook that you can run some check before publishing is allowed. Paid member, or content or rate limit, etc.

I don’t know of anyone else using it, however I wrote a couple ML models and have a decent dataset for training. I use it successfully in production - however it only works for events with enough content, not reactions, reposts, meta, etc. Bayes model works with fewer resources than the FastAI model.