>> to ISO-8859-15 '¼', which is clearly wrong.
>
> Fixed.

Yes, thanks a lot.  However, I think you should unifiy to ISO-8859-1
instead of ISO-8859-15 because the standard utf-8.el can deal with
ISO-8859-1, but not ISO-8859-15.

BTW, there is a typo in the docstrings for 'mm-hack-charsets', and the
docstring for 'mm-iso-8859-15-compatible' seems to be a bit unclear.