It seems like the problem with doing this is that there are multiple localhost IPs. If it was as simple as "every A query for [any].localhost is 127.0.0.1, ever

It seems like the problem with doing this is that there are multiple localhost IPs. If it was as simple as "every A query for [any].localhost is 127.0.0.1, every AAAA query is ::1" then it would be one thing, but the A query could also be 127.0.0.2 etc. So for example this would mean that if I put an entry for "server5.localhost 127.0.0.5" in my local DNS server then that entry can't be used, because the client's local resolver API will match *.localhost and it now "MUST NOT send queries for localhost names to their configured caching DNS server(s)" so it likely returns 127.0.0.1 instead of 127.0.0.5.