You can connect through a DNS, and lots of servers utilize this. (I'm actually impressed you haven't seen any, since this is how fragshack and many other huge name servers operate.) However, how would connecting through a DNS allow the redirecting of traffic to other servers? Can I write a linux script to do it?
EDIT: Or maybe something with IPTables? <If player attempts to connect to port 27025, check such-and-such script. If script echoes player count greater than 19, direct traffic to port 27026>?
Ah, didn't know. Never really played any community server other than AM. How come we haven't set up arena.absurdminds.net? It isn't hard to setup an SRV record (domain:port) for this.
Mainly you are requesting the port and you will be sent straight to that service. I don't know if it will allow the server you are connecting to to redirect you. IPTables does work for things like that, but its a matter of if it breaks the client connection.
That being said, I could screw around and actually test this by setting up my own domain stuff with my server and then playing with IPTables.