Also for the record I've now gotten AFKMud to compile under Cygwin again (damn libraries) and confirmed that even on my Windows 10 Home PC that the code bound both addresses and accepted a connection from another PC in the house. Neither of which get assigned IPv6 addresses because our router is so backward it doesn't even support it at all.

I don't think portability is an issue here. It's just a matter of getting the DNS resolver process to play along now.