To use their authentication tag, I would follow their instructions and paste it directly in the head element in the layout.master file of the skins used on your site. The built in support for meta elements is for use on a page by page basis and isn't designed for the meta element that bing wants for authentication.
The best approach would be if you know someone who can access the server then have that person drop in the auth.xml file and be done with it.
Hope that helps,
Joe