Instead, unlike unusablly long encoding with multipart/mixed-charset
switching, ISO-2022-INT-* can be used anywhere inside or outside of
MIME.
Sorry, I don't have a single solitary device or piece of software that can
deal with ISO-2022-INT-*.
But you can have it.
So your solution CANNOT be used "anywhere inside
or outside of MIME"; it can only be used "inside ISO-2022 systems".
Unlike ISO-2022-INT-* encoding which can be used anywhere, you can't
use multipart/mixed-charset outside of MIME.
Masataka Ohta