There's not much difference between these two, as a channel operator can ban/filter addresses and an IRCop can alter the server to return basically anything. For both you need to trust the maintainer not to tamper with the service.

If you want the clients to dynamically add their addresses to the bootstrapping service, you can do this with DNS, too. E.g. set up a DNS server that accepts DNS UPDATE (RFC2136) to add new records (obviously deny delete attempts and purge outdated entries from time to time). Or set up a tiny UDP update service additionally to DNS where clients can submit their address to the database.

You could ask for TXT records which store addresses in the same format that are used for IRC nicknames.