What exactly do you mean by "support"? Do you mean the ability to
convert to another character set in the list?
I mean simply the ability to display the specified character set.
Is this really the province of an 822-like RFC? 822 seems to me to be
more like something that specifies the format of messages, including
the header names, how to write headers, etc.
On the other hand, I can fully understand your desire for
interoperability. But I don't think we can really expect everyone to
have all fonts. There will always be at least a small number of people
with dumb terminals that can only display e.g. ASCII. Or people with
1.5 Megabyte RAM X terminals, that can't display Japanese.
When people create standards, they usually try to limit the specs to
something that is implementable, both technically and politically.
From this viewpoint, if we want to specify that the characters be
displayable, we would have to limit the character set to the least
common denominator, probably something like the invariant set of ISO
646. Now, I ask, is this really what we want? (It's not what you want,
you even mentioned Unicode!)
So, I think you should relax the displayability requirement, and just
say that the UAs must be able to interconvert between the listed
codesets. Of course, there will be messages that cannot be converted
to one of the desired codesets. In these cases, I would suggest not
converting at all, perhaps telling the user that it couldn't be
converted.
Erik