Rich Salz and Dave Orchard have written a proposal for a URN scheme for
XML QNames. Basically they propose that a qualified name such as
xsl:template could be written as urn:qname:xsl:template:http://www.w3.org/1999/XSL/Transform. The default namespace can be handled by omitting the prefix. For examle the XHTML p element would be urn:qname::p:http://www.w3.org/1999/xhtml. You can ignore the prefix by using an asterisk. For instance, urn:qname:*:rect:http://www.w3.org/2000/svg matches both svg:rect and rect in the default SVG namespace.
I've expressed a few technical and editorial quibbles with the current draft to the authors, mostly revolving around the distinctions between URI, IRI, and URI reference,
but nothing fundamental.
Overall this seems like a pretty solid idea.
I'm not sure exactly where this would be used, but it sounds like it ought to be useful somewhere.