Well, again, I think that Dave's suggestion is a change of syntax only,
and therefore I don't feel too strongly about it either way.
I do belive that "message" is a good place for external reference, for
the same reason that I believe it to be a good place for "partial",
namely that it provides us "for free" with a mechanism for typing the
external or partial object. That is, the re-use of the mesage object
means that we don't need to define any new mechanism for content-type,
etc.
The only specific problem I have with Dave's suggestion is that it means
we will have to add "applicaton/external" to the list of types that
can't be encoded. Currently, that list is just "message/*" and
"multipart/*", and I'd like to keep it simple...