I'm using Shibby's 1.28.0000 MIPSR2-105 K26 USB Big-VPN if that matters.
The screen to enter static host information works great for creating IPv4 hosts, however there isn't enough space in the input box for IPv6 addresses. Even single IPs on /56s don't fit.
From testing by editing the configuration files directly (which doesn't survive a reboot) it would appear that simply making that field larger would entirely solve the problem. Dnsmasq appears to handle IPv6 addresses in those fields just fine.