http://slate.msn.com/id/2102724/

this article seems to suggest that any kind of donation, if you don't have U.S. citizenship, is out of the question

And it's pretty much too late, at this stage, to do much else. I wonder if you could contact someone in the DNC (or some other political party, or a group like moveon.org) for future elections, offering to do some sort of volunteer work for them. As long as you're not compensated with money, this article suggests it's legal.