You know, I can't manage to send HTML mail from mutt. Gnus (and I think all of the other emacs mailers share the relevant code) I'm pretty sure I could do it in, one way or another.

Failing that, I could always try to compose the message by hand, but that'd be the kind of HTML mail with no text part (to which, on the receiving end, I have not been kind) unless I felt like some RFC-diving.