There is no technical difference between regular IPv6 addresses and Unique Local Addresses (RFC 4193
https://tools.ietf.org/html/rfc4193). You can use both at the same time on your LAN without any problem. To prevent ULA's from leaking to the outside world, the RFC instructs to block them at the site border:
Site border routers should respond with the appropriate ICMPv6 Destination Unreachable message to inform the source that the packet was not forwarded. [ICMPV6]. This feedback is important to avoid transport protocol timeouts.
In RouterOS this is achieved by adding this line to the routing table:
/ipv6 route
add comment="Keep ULA's inside" distance=1 dst-address=fc00::/7 \
type=unreachable
Wouldn't it be feasible to take the same approach when it comes to IPv6 WAN failover? Just advertise and use the prefixes from both ISP's on the LAN, use policy routing to force each prefix over the correct ISP and block at the border router the prefix that shouldn't be used at that time? It probably takes some scripting to detect the failover situation and to subsequently change the corresponding rules in the routing table.
I haven't tested this setup yet, but I suppose it could work this way without too much overhead, at least if your site isn't too big. I think I'll give it a try later this or next week.
