I have just tested this on an IPv6 connection and mIRC is receiving, and replying to, Identd requests when connecting to an IPv6 server address for both Rizon and Freenode.

mIRC does listen on both IPv4 and IPv6 sockets for Identd and DCC Server connections.

If it is not working for you, the most likely reason is that your NAT router or firewall are not configured correctly.