Use Windows DHCP/DHCPv6 server without setting a static address
Solution 1:
Now I know I said setting up a ULA was not an option but it is actually far easier than I thought. You do not need to publish the prefix. And no other address in the ULA subnet even needs to exist anywhere.
Yes, it is a garbage address that clutters up the routing table but at least it doesn't get into other machines.
Hopefully Microsoft will take note at some point and allows their DHCPv6 server to respond on other available addresses besides static. It makes sense to require a static address during stateful address assignment in order to reliably map scopes to interfaces. But it does not apply to stateless DHCPv6.
Unfortunately, there is no other answer to this question.