Per iCalendar, you can only set CHARSET at the content-type header level, not at the calendar component level. This was removed from vCalendar, as it was massaged into iCalendar. Repeatedly, I have received requests from pacific rim reviewers of iCalendar for this function. But, I believe that the IESG philosophy is to set the character set at the MIME entity level, not within the the MIME entity.
-- Frank