So, iOS has a "broken" mail client: it doesn't show messages
  that are forwarded as MIME without a MIME-decoding/MML step.  So if you

I have noticed this too.  Do you mean

  attachments of type message/rfc822 are not displayed

and you are suggesting sending a message which is multipart/related and
has a text/html or multipart/alternative type instead of a
message/rfc822 type?

It sounds like gnus could grow a 'workaround-broken-ios' variable that
does forwards with a mime type that ios can handle, perhaps.

FWIW, another coping strategy is to resend rather than forward, but I
realize that's lame in the general case.

Have you filed a bug report with apple :-) ?