Internally we use the tags field to track which builds are associated with which baselines.  Also we mark the builds which are milestones via the tag.  I don't think that there is a way to search other than via the tag.  I believe the reasoning is that there is so much churn on builds that it doesn't make sense to index that information.  Rather its better to have an "opt-in" search capability for the important builds that have been tagged.  There isn't a way to automatically tag a build that I know of.  That might make a good enhancement request since it seems like there should be an ant task that could do that for you...