At 4:14 PM +0000 12/8/04, Jan Egil Kristiansen wrote:
>I'd like a DOCTYPE to make it possible to validate XHTML with 
>well-formed extensions from any other namespace.
>
>See http://styrheim.weblogg.no/081204103845_the_x_in_xhtml.html 

One good place to start learning about the work that is ongoing in
the W3C in this area would be the work of David Orchard for
the TAG, in particular the three links provided in:

http://lists.w3.org/Archives/Public/wai-xtech/2004Dec/0000.html 

The Technical Plenary 2004 session on experience with mixed
namespaces is also instructive:

http://www.w3.org/2004/03/plenary-minutes#Session4 

Down the road, you should also watch the progress of the compound
documents working group:

http://www.w3.org/2004/CDF/ 

The formation of this group gives evidence that this is an important,
and under-solved problem.

I don't think that they are going to produce anything of the sweeping
generality that you asked for, but what they do produce is likely to
be helpful for your actual problem.

Al