Steve,
You're right that 1421 makes an explicit mention of the
syntactic ability to intersperse PEM and non-PEM body parts in a
single content. As I noted, there were concerns about user interface
issues and so we downplayed that potential facility and I don't
believe any implementations support it in any automated fashion.
Long ago it was a shared goal develop a version of MIME-PEm
that would be easily processed by users without full MIME capability,
and the first few drafts, I recall, tried thatb approach without
success. That idea also motivated Jeff's proposal, which he later
rescinded. The current I-Ds don't seem to consider this feature,
and so I assumed that we had given up on that tact.
Steve