> Do providers like NextDNS or RethinkDNS allow these sorts of overrides
Not on the resolver, but on the Android client (Rethink DNS + Firewall) we do (if enabled) manipulate DNS answers to implement an opportunistic 464xlat (over Kasper Dupont's public relays [0]).
You'll need to update your DNS server to include those as AAAA records.
Do providers like NextDNS or RethinkDNS allow these sorts of overrides?