The threat model on this big enterprise WAN isn't "an authorized user creates a VM on an existing machine", but rather "some rando plugs something in with an Ethernet cable and goes to town on the intranet services". It has a whole web portal where you can log in and register your devices. So it's not like anything's really being bypassed here.
Anyway, I still prefer NAT for VMs over most passthrough schemes, which I've found unreliable even on IPv4 networks. What the outside network doesn't know about can't hurt it.
Isn't that what the MAC filtering is for? I don't see the point of stopping a single registered MAC on a single port from having multiple IPs.
(If I had to guess, just letting any machine have a million new IPs for the firewall to track has its own issues, so you'd end up with policies upon policies.)
In any case, sometimes middleboxes just don't behave precisely how we want them to, and that's why I'm skeptical of the typical IPv6 position of "a flat /64 network (or something emulating one) is all you'll ever need".
And sure a million IPs could cause problems, but that's not a good reason to set the limit to 1.
> I'm skeptical of the typical IPv6 position of "a flat /64 network (or something emulating one) is all you'll ever need".
That's not the position. You can have as many networks as you want. Connect them with routers like you would on IPv4.
Contrary to popular belief, IPv6 does have NAT, it's just stupid and unnecessary in most use cases. There are even different kinds of NAT; there's the "swap the public prefix with a private one" NAT (useful for shit ISPs that rotate your IPv6 prefix to make you pay extra for a static one, so your addresses don't keep changing), or the IPv4-like "masquerade all traffic when forwarding" one that'll work in your weird enterprise network.
With the latter option, you can set up an IPv6 with DHCPv6 on the upstream and whatever combination of SLAAC or DHCP you want on the VM side, and set up NAT using guides like this (possibly outdated iptables-based) one: https://forums.raspberrypi.com/viewtopic.php?t=298878