Both IPv6 site-wide multicast and unicast packets must be passed between the stub and infrastructure networks in order to support all host services including (discovery through mechanisms documented in RFCs 6762 and 6763) on the stub network.
I don't really know what it means to discover a host on the Internet through those RFCs, btw, nor do I think that's what you intend, but that is the implication; since you can't possibly mean that the upstream must support multicast (they often don't).
This section takes a lot of text to state:
I don't really know what it means to discover a host on the Internet through those RFCs, btw, nor do I think that's what you intend, but that is the implication; since you can't possibly mean that the upstream must support multicast (they often don't).