From: Dave Abrahams <dave@boostpro.com>

  When I have a message with Content-Type multipart/alternative containing
  HTML and text parts, I'd like to be able to easily switch between
  viewing the text parts and the HTML parts without showing buttons,
  switching to the message buffer, and choosing the parts in question.

I wanted to do the same thing (to get a link that was missing from the
text/plain part), and found the following, which isn't quite what you
asked but was good enough.



W M v runs the command gnus-mime-view-all-parts, which is an
interactive compiled Lisp function in `gnus-art.el'.

It is bound to W M v, <menu-bar> <Article> <MIME> <View all>.

(gnus-mime-view-all-parts &optional HANDLES)

View all the MIME parts.