I still think I have an easier solution. Actually the judicious use of MFN status might be a way to implement my idea.

We have worker protection laws like OSHA and the like. This is because we don't think people should be treated certain ways. So it isn't ethical to buy prodcuts made in those ways, is it?

So just prohibit the sale in the U.S. of any product that the manufacturer can not verify was manufactured under the same worker safety (and other) guidelines as U.S. manufacturers must follow.