I can get the email to take, the HTML/txt alternative block, I can base64 encode the image, and I can get them all to appear in the delivered email, which seems happy to display txt or HTML as appropriate.
But trying to load an img tag with a cid: src is just beyond me. Is the Content-ID supposed to be globally unique or summat?
Update. Ah! multipart/mixed != multipart/related. D'oh.