I've always been able to solve this problem more simply using IPv4 link-local addressing (https://www.rfc-editor.org/rfc/rfc3927), it doesn't matter if somebody abuses the same range because the outbound interface is explicitly specified in sin6_scope_id passed to connect() and bind().

But if you're writing code at a higher level than the bsd sockets api, it's a whole can of worms.