I think the problem is in using reserved IPv6 address ranges (in particular, the 0100::/8 block), so ip -6 route won't let you use it as a gateway. (See »www.iana.org/assignments/ipv6-ad···pace.xml)
My guess would be that the FC00::/7 block would be appropriate (see RFC4291), or something in 2000::/3 if you don't mind clashing with the global IPv6 space.