if(!cr_route("default", "1", "$rU", "$rU", "call_id")){
    if(!cr_route("default", "2", "$rU", "$rU", "call_id")) #here rule 3 would be selected because domain is 2, not 1
        send_reply("403", "Not allowed");
}

So you can have a special domain for each carrier that has no flags set and just acts like a catch all construct. Performance should not be impacted.

Hope this helps.
Marius