Well, there are some US policies that do provide world wide liability coverage on rental vehicles. You may not be able to purchase a policy like this, but one that I am aware of is the Chubb High Value automobile policy.

Finding the coverage for free on a credit card (or even low cost) is going to be next to impossible. Realize that on CDW, the potential risk to the car issuer is maybe $50,000 max. Liability coverage, if offered, with be useless to most people if it provided less than $300k coverage - something that I doubt the credit card would give for free or even at low cost.