David:
Please see http://www.ietf.org/rfc/rfc2396.txt.
Section 1.2 URI, URL, and URN in the above document states:
A URI can be further classified as a locator, a name, or both.
Thus, a URN is also a URI. Therefore, I don't see any problem
with the URN we have chosen for the nextMSH and toPartyMSH.
Regards,
-Arvola