If URNs are allowed to refer to multiple formats of documents, or
multiple versions of updating documents, or online streams of
information that you might telnet to, then no, you can't easily
include the MD5 signature of the document.
What you're saying is that you don't have to trust the URN -> URL
resolution process, because you will verify the entire URN ->
I'm not sure I can live with that, although I'd like to think about it
a bit more.