Fair enough. Maybe using a NDP proxy daemon (ndppd) like mentioned in the documentation could do it as workaround:
Linux have a limited support for proxying Neighbor Solicitation
messages by simply answering to any messages where the target IP
can be found in the host’s neighbor proxy table. To make this work
you need to enable “proxy_ndp”, and then add each single host to the
neighbor proxy table by typing something like:
ip -6 neigh add proxy <ip> dev <if>
Unfortunately, it doesn’t support listing proxies, and as I said,
only individual IPs are supported. No subnets.
‘ndppd’ solves this by listening for Neighbor Solicitation messages
on an interface, then query the internal interfaces for that target
IP before finally sending a Neighbor Advertisement message.
You can create rules to query one interface for one subnet, and
another interface for another. ‘ndppd’ can even respond directly to
Neighbor Solicitation messages without querying anything, should you
need that.