OTOH, rewriting of source locator is not so useful, though I have
no reason to forbid it.
Unless the source address is already a valid address assigned by the
ISP you're forwarding the packet to (which can't by definition always
be the case in a locator/identifier scheme), you need to rewrite it in
order to get through ingress filtering and to be able to receive ICMP
messages. Remember that path MTU discovery is pretty much mandatory in
IPv6 so you need those ICMPs. Ingress filtering is important to keep
denial of service attacks in check to at least some degree.