You shouldn't use MIME charset="xxxx" parameters that haven't been
registered with IANA. Currently, the official list of MIME registered
character sets is in
ftp://ftp.isi.edu/in-notes/iana/assignments/character-sets
In any case, I think the HTML 2.0 spec can get away without
recommending user agent behavior with respect to charset parameters,
as long as the SGML document character set part is settled.
You might still want to put something in the text/html MIME
registration, though.