I've kept an eye on Microsoft Tag because I think it's an interesting project and has a lot of potential. Recently, I visited the site and was surprised to learn that custom tags are now supported! I would recommend watching the following video, which explains how to create your own:
Based on the video description (above), I was able to create an ugly-yet-usable tag for the Canadian Developer Connection blog:
In order to try this out yourself, point your mobile device to gettag.mobi to get the reader software (which supports many different platforms). From there, you can create your own tags on the Microsoft Tag site.
Finally, take a few moments to check out the Microsoft Tag Development Center if you're interested in bulk creation of tags and other operations.