Ok, so I found out that what I want is "stateless DHCPv6". I guess this is beyond the capabilities of netsh. And I don't want to install a server OS either. What I found is "dibbler". This claims to support stateless DHCPv6. This is my dibbler server.conf:
iface "VirtualBox Host-Only Network"
option dns-server 2001:470:20::2
I just want to promote the DNS-server address. But it doesn't work. It doesn't see any requests from the linux guests, and the windows guests do "soliciting", and dibbler complains that this is a stateful request and therefor ignores it.
I am not sure and it is not very clear: is dibbler a server/client-thing? So do I need to run the dibbler-client on my guests? Why is it called DHCPv6 then? Because if it needs the client as well, it is something that just does the same, but in its own protocol.
EDIT: Aaaah, I am very sorry. I found the solution! The interface that is sending the RA has to tell the clients that "there is more", i.e. that it is not plain SLAAC, but that there is also stateless DHCPv6 (commonly referred to as the 'O'-flag). All I had to do is issue this command:
netsh interface ipv6 set interface "VirtualBox Host-Only Network" otherstateful=enabled
And then have dibbler run and the clients successfully take the DNS-address as advertised by dibbler!