I'm far from being sure, but if I wanted to use Org as a mean to write
HTML or PDF as message attachments, I would think it could be done with
a very moderate amount of Emacs Lisp customization.  Unless mistaken,
the Org editor to HTML might yield its output in a temporary Emacs
buffer, (for PDF, maybe not) and I would be fairly surprised if message
composition does not have facilities to receive attachments from
buffers.  And even if one would have to go through an external file, the
extra overhead might be negligible in practice.  Putting all the pieces
together might require some work, but maybe not that much.