I don't think I understand how what you're asking for is different from /server -46 or /server group .
You can create a server group, one server with your IPv4 address and one server with the IPv6 address. If you connect using /server group , then mIRC will cycle through your list of IPv4 and IPv6 addresses until it connects.
Another option is to modify your hosts file. In Windows it's at C:\Windows\System32\Drivers\etc\hosts. Then you could add your IPv4 and IPv6 using some name:
192.168.1.1 mylanadapter
fd00:dead:beef:cafe:: mylanadapter
and then you can just use the address "mylanadapter" in your mIRC configuration or /server command.