ACLs in relays are part and parcel of what NIP-42 is about. in the nip it even mentions the concept of blocking access to a privileged type of event, a DM. i've built a simple ACL that uses follow and mute lists but the same principle would apply to add checking and caching nip-72 room definition events, you could have both on a relay, and override the check to use these instead of the plain subscriber list for the relay