I think it would be nice to support some "magic" MIME type that the filter could return to indicate that no alternative should be generated. I may want to use this for an occasional message, but not for the vast majority of messages that I send. I'm sure that setting the option to ask-no, would get annoying fast; but I suspect that I could figure out some way for the plain-text content to indicate to a filter that I want alternative generation.
The first thing that I tried doing was generating a multipart/related alternative with HTML and an image. That resulted in a fairly messed up message. The MIME headers for that alternative seemed to come through correctly, but the content was reduced to just the ending boundary line. Viewing that in mutt caused it to treat the multipart/related content as text/plain. I wasn't really expecting this to work, but the failure mode was definitely unexpected.
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