x-gzip64 みたいなやり方も無いではないが。No, the approach proposed in that message just doesn't make any sense.
It's still using magic text strings.
yEncode as a content transfer encoding is not at all incompatible with
MIME. It's actually quite straight-forward. Take the current yEnc
encoding rules, strip out the needless begin and end markers, remove the
checksum in favor of using Content-MD5, and call it a content transfer
encoding of x-yencode using the standard MIME encoding rules. All done.
As a content transfer encoding, there's no need to worry about multipart
messages, interaction with other MIME parts, or any of that. It just all
falls neatly out of the existing rules for content transfer encodings.
Any MIME part that's encoded using yEncode has in its header section the
line:
Content-Transfer-Encoding: x-yenc
and the body of that part is yEncode-encoded information.