How to use the build tag?

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...