Actually, if you take a look at RFC 2396, which outlines the structure of a URI, you'll note that mailto: and news: are exceptions to the general rule regarding schemes. While Section 3 does say "The URI syntax does not require that the scheme-specific-part have any general structure or set of semantics which is common among all URI", I think you'll notice that the next sentence offers a bit of advice:

However, a subset of URI do share a common syntax for representing hierarchical relationships within the namespace. This "generic URI" syntax consists of a sequence of four main components:
     <scheme>://<authority><path>?<query>
each of which, except <scheme>, may be absent from a particular URI.

Since the prefix tags are meant to fit within our own namespace, I don't see any harm in keeping them consistent with the majority of generally-supported schemes listed in Section 1.3.

After all, if you're used to typing "://" as part of your UR(I|L|N)'s, why invent an alternate syntax for our local schemes? Consistency can be a good thing, after all.

So, I vote 'No.' Sorry...

--f

P.S. BTW, Appendix B of that same RFC offers an interesting discussion for anyone trying to use regular expressions to parse URI's. Of course, I imagine that URI-1.17 (among others) would be helpful, too. :-)