Never via that particular site, but looking at Dollar and Thrifty's own UK sites, I've noticed the T&C have no specific prohibition on a US resident reserving a car via those sites.

Since I often take the rental company's coverage and it's far cheaper to book via the UK-based prepaid sites, I've been tempted in the past to reserve through one of those sites and get the whole bundle for less than booking directly as a U.S. citizen. I have not yet gotten the courage up to try, however...