Actually, Nathaniel, I was planning on implementing just this feature
in the external reference stuff, but only for local use. Since by definition
such a reference cannot leave the local system, and MIME restrictions
on nonstandard additions don't apply locally (as long as they're eliminated
before a message passes out of the local system) I didn't see any hard in
doing this.
My use for it was specifically as an internal thing within an MTA, to
avoid making multiple copies of message bodies when they are not needed. It
gets very complex when conversions are going on, but even so the
savings for large messages can be substantial. My only problem is the
implementation of reference counts so I know when to get rid of the
body, but this is strictly an internal matter for me.
But I certainly don't have _any_ problem with adding this to MIME proper.
We have other stuff in MIME that addresses local needs only.
Ned