Ugh- I'm so in your boat right now! Just finished my MSW in june, applying to medical/psychological anthro programs with emphasis on science/technology studies and feeling VERY SOL re: writing sample options. The longer papers I wrote last spring with the hopes of using for applications are not quite topically aligned or as good as they need to be, and one other option is only 8 pages, and from my first year.
I don't have a lot of feed back except to say, I feel you, and if its any consolation, unlike my partner in philosophy who got into programs I think mostly on the base of his writing sample, I've had faculty tell me (somewhat sheepishly) that no one is "really" going to read that much of it, and they are much more concerned with the research project outlined in your personal statement.
So, I've been trying not to forget that.