It is a clever idea, and might be worth considering, but it doesn't feel natural to have to specify the attachments before the html.  What I'd really like is an API that hides the messy details of the RFC from the library user, and I haven't thought of anything I'm satisfied with yet.  Specifically, it would be great if the 'keys' could be specially formatted strings ("@mypicid1@" or something like that) and the helper would automatically replace them with cids in both the html and the attachments.

It would probably be worth taking this discussion to the email-sig mailing list.