Mark,
(I do think that this topic belongs to the ietf-xml-mime ML. I am
ccing to the ietf-types but will not do so any more.)
I think that you have presented some requirements which are not
addressed by any of the existing mechanisms.
But I believe the security problem is just one manifestation of the
layering problem inherrent in using intrinsic dispatch (of which XML
namespace dispatch in one kind); that an extrinsic dispatch mechanism,
like media types, cleanly separates the "what" - the raw data - from
the "how" - the semantics with which that data is to be interpreted.
I do not think that media types are good enough for multi-namespace
documents and that W3C should develop something different. Although
your example can be addressed by specialized media types, my example
(schemas embedded within XHTML are referenced by XPointer) cannot.
Cheers,
--
MURATA Makoto <murata(_at_)hokkaido(_dot_)email(_dot_)ne(_dot_)jp>