That's already done. If you're big enough that renumbering is a concern, but small enough that you don't have your own assigned prefix, you use unique local ad

That's already done. If you're big enough that renumbering is a concern, but small enough that you don't have your own assigned prefix, you use unique local addresses (fc00::/7) and NAT them to your providers space. (Which works better than it sounds - because you have enough space to nat one-to-one, the translation mechanism doesn't need to keep state. Simply replace one prefix with another, whack a new checksum in and send it along)