Could it be better to include the domain in an 'r' tag for example than in the content? This can be more queriable using filters
DNSTR - Domain Name Mapping for Nostr Public Keys
https://gist.github.com/melvincarvalho/09b8d8dfdb6d89a9043fae26e9312b1e
Discussion
r is tricky because it means both relay and reference
what i think is actually in the content is an origin
but you are right, it may be better as a tag -- this is just a draft of a draft