It's an interesting idea, but I suspect that the people losing their homes in the US thanks to shoddy mortgage marketing practices would have more of a case than US expats. The problem for the expats is the drop in the value of the dollar - and that's a well-known hazard of expatriation.

If the mortgage holders can't manage a class action suit, I think making the "direct" connection between sub-prime mortgages and the freefall in the value of the dollar would be a stretch at best. Besides, there are plenty of folks in the government who are thrilled to have the dollar at such a low point - it makes US exports much cheaper on the world market.