RFC 3306 [RFC3306] defines a way to derive an IPv6 multicast address from an IPv6 unicast prefix.

If someone needs an IPv6 multicast address, he/she can derive it from its unicast prefix. There are potentially 232 addresses per /64 prefix (per link). The question remains about the way to choose the last 32 bits of the prefix-based address. Nevertheless, the allocation problem is now on a smaller zone that is known and managed.