temporary urn:predicates

I was talking recently about barriers to producing semantic web data.

Normally a predicate has to be

- A URI
- Preferably an HTTP URI
- Preferably an existing URI

This (Im told) can be a barrier for newcomers.  They have to find the right
name for a predicate, the right URI, and then see if it's already used.  If
not create their own vocabulary.

At this point some might give up.

So I was wondering how it might be possible to create a temporary URI that
people could use as a place holder, so the software still works, until they
think of a better name.

We've all had to do this at some point, right?

So I originally used to use things like:

</predicate> or
<#predicate>

But that breaks down when you start using multiple documents because the
URI is relative to the base.

So I thought why not use:

<urn:predicate>

It seems to me local predicates are "just a name" and a urn is "just a
name" so it would be a good match.

My main concern is whether it would have collisions with the IANA registry:

http://www.iana.org/assignments/urn-namespaces/urn-namespaces.xml

Any thoughts?