EXMH actually implements multipart/alternative by showing the "best"
part and prefacing it with a line saying "use right button to view
alternatives"; the right button brings up the list of alternatives and
lets you try to view one of the others.
EXMH also is the only mailer I've seen (apart from Andrew, which I
haven't seen, it seems) that follows a pure "everything is inline"
strategy, so I *do* know that the stuff I was looking at was a
multipart/alternative for the second part of a multipart/mixed.
Hint: The IETF internet-drafts announcements are examples of this