I'm not sure about the complexity since it's done for tcp but you might be right.
Of course mIRC can be changed to support hostnames here, but is it really worth it?
I have a client and a server where only the server needs to open a tcp port and a udp port.
Since the server can run with any tcp/udp ports, it's up to the client to specify an ip address and a tcp port, the client first connect to the server with tcp and then udp is done internally (the server send the udp port).
In this case when the client want to send a packet, it will use the ip address specified when connecting with tcp, and it will fail if localhost has been used.
I could do $iif(address == localhost,127.0.0.1,$v1) but it doesn't seem to be the good solution so imo, yes.