This morning I managed to not only make it to Starbucks (albeit a bit wet), but complete a small scheduled task script which grabs all of the Messenger tag lines I've used in past days. It works by enumerating all the XML history files, parsing them with RedXml, matching User nodes' FriendlyName attribute against a RegExp, and then keeping these sorted by date/time. After each pass on a single XML file, a clean-up loop runs which removes duplicates and leaves only the "first known" instance. Thus, in almost all cases, I end up with the correct tag line for a particular day.
I even managed to find some XML files from 2005 and I've got over 250 tag lines stored. These will be browse-able on my blog site (www.masta.us) at some point, along with my avatars.