I think that several of us are watching the URx work very closely
and that it is reasonable to expect some external-body types for them as
soon as that work stabilizes.
From my point of view, the level of thrashing around in that discussion
right now--about character sets, about the syntax for the name of a mailbox
for that type of reference, and so on--makes our doing anything in the
MIME context quite premature.
On the other hand, if/when the URx work comes to anything that can be
used operationally to retrieve documents, it would be really dumb to
not have an external reference type in MIME that can use them. I would
hope we could avoid "really dumb".