It's possible to do what you describe with existing open-source/open-hardware U2F security keys, as they let you define the master secret from which the per-sit

It's possible to do what you describe with existing open-source/open-hardware U2F security keys, as they let you define the master secret from which the per-site private elliptic keys are generated. I haven't personally tried it, though. But for security you should still have at least two U2F security keys using different master secrets (if you lose one security key, you can deactivate it without deactivating the other).