I don't know if it's a bit crazy, but what about installing an instance of nostrcheckme server and implement this part for nostr.buid? There are several endpoints, and you could use only the NIP98 part (or whatever you want of course). In fact my idea is that the software can be used for any backend in nostr, reaching the final goal that any user can install it in umbrel for example.
Then the clients should only have a section to specify the URL of the backend server (multimedia, etc).
I totally discarded PHP to advance (a little) to the rhythm of nostr, because otherwise it is impossible 😅
Another advantage, Amethyst already has it implemented for nostrcheckme server, changing it for nostr.build will be very easy for them.