The "experts" all say to never use NAT66 because it's EEEEEVIL, but it would solve my use case in a much simpler manner than having to do both SLAAC and DHCPv6 on all my subnets and both protocols on each client node. I'm pretty sure I could more easily get NAT66 working on my gateway router, but I'll try doing this the hard way, just to learn how to do it.