Official response from Adriel Hampton
Noah, I'm not sure exactly what you're asking for, but if I get the gist, consider that NationBuilder handles complex queries using a shoppping cart for your lists. Add all the people with a certain tag, subtract all the people who've already signed your petition, subtract all the people who've got another tag that you've already blasted to, etc. Save list. Here's a screencast on using the shopping cart to design and targed complex lists with NationBuilder.