It's true that the potential for protectionist uses of that carbon tax or tariff could be a problem, particularly when you're imposing a tax on the embedded carbon content—and it's often hard to trace exactly what that is.
There is a precedent for this in the U.S., I think, with the import tax they imposed on ozone-depleting substances, when they had a tax on ozone-depleting substances to help phase out those substances. I haven't looked at all of the trade law on this, but I think it satisfied World Trade Organization standards. It was based on a set of presumptions about the embedded ozone in products. So to be comprehensive, you'd have to have some kind of a tax that would have presumptions about the embedded carbon content in various products.