Between the answers posted to this list and the answers sent to me as
mail, it's obvious that none of my suggested alternatives was good
enough. The real problem, I think, is that the current default
content-type for mail has never been well-enough defined for any of the
more specific terms to properly apply. Thus, for example, calling it
"NVT-ASCII" or "IA5" might actually be strengthening the constraints on
it. While that might be desirable, it opens a major can of worms, and
was not the intent. What I'm seeking is just a descriptive term for
"what mail bodies are now assumed to be."
How about simply
Content-type: US-7-bit
which is, at least, a non-technical term that somewhat describes the
status quo....