You can. I've seen sites use it because they didn't have an allocation of their own, but they didn't want to rely on their ISP's numbering either. So they used

You can. I've seen sites use it because they didn't have an allocation of their own, but they didn't want to rely on their ISP's numbering either. So they used 1:1 NAT between their carrier's space and their ULA space. Like renting a /8 and mapping it to 10/8. Except in v6land it's nowhere near as crazy as it sounds. NAT isn't required in v6, it's not wanted in v6, but it's perfectly possible. If you think about it, PAT/NAT (port-based NAT) doesn't happen at the IP stack, it happens at the port level.