Perhaps not specialized relays that do a few functions well, but an additional feature for existing relay architecture. It needs the database for effecient storage and retrieval, so a relay that expects embeddings as a feature should use this "module" to know what to do with these event kinds. Otherwise, block the event because you don't want to be holding embeddings unless you actually care about them. 

with that, you can automatically embed events as they come in - or you can have an npub be an agent to compute embeddings as you need them. I'd expect embedding all kind1's flowing through a relay, or even a set of relays to be intensive, so you optimize in the quality of the content you embed. User's calling a bot for the job is definitely a signal of quality.