There have been a couple comments in my feed from people expecting to paste a nostr.build address so that could be cool

Your API is fast already, it's mostly that nip98 AUTH requires me to publish a Kind 27235 note, which takes time to broadcast with my current library

Were you thinking this API would be separate from nip98? 

 Nip98 does not require anything, just a signed header. You are talking about nip94, that can be pushed async.