I'd be tempted to make all external references be of the form
[ref: some-content-id], where the content-id must refer to a
content that appears in the same message as the reference. The
content-id could, however, be that of a message/external-body content...
that way any extensions to the MIME external reference mechanism are
automatically incorporated into simplemail. Mail readers would have
to provide hooks for this, but I'm sure other content-types will want
to be able to do this kind of referencing also.
Keith