I responded to a question about RDDL microformat on www-tag.
Like many other folks, I've been interested in mformats. I'm not sure
about the utility of RDDL as a mformat, though it would be easy enough
to write up. BTW, I think your proposal is a little more than a mformat
for Xlink than for RDDL. I think the big upside to mformats is
embedding structured information in a lot of web pages, like
reviews/cards/events, etc. and then having this information extracted.
I don't think that are that many RDDL pages that are out that there that
would be extracted for searching nor are the queries that interesting.
I see a big use case in software engines like Technorati or kritx or
whatever extracting and then doing something with the info. For
example, the scenario I'd love to see is a google/yahoo/msn map that
shows all the 4/5 or 5/5 rated restaurants in Vancouver .
Now I can't see the equivalent use case for RDDL. In my mind, the main
use case for RDDL is finding information about a given namespace name.
That is, the namespace name is a specific thing in hand. Now imagine an
engine that grabs RDDL docs. I don't think there's much utility in
asking questions like "find me all the namespaces that have RelaxNG
schemas associated?".