OTOH, if the message is structured
multipart/related multipart/alternative text/plain text/html image/png image/png
the receiver can infer that the images are related to both text/* parts and DTRT for each.
With the images being treated as attachments. Or is there a syntax to allow the text/html to embed the images and the text/plain to see them as attachments?
I believe the above is that syntax. But the standard doesn't say anything about this. The standard for multipart/alternative is RFC 2046, which doesn't know about multipart/related. RFC 2387 doesn't update RFC 2046, so it doesn't say anything about multipart/alternative within multipart/related, either.
I think the text/html wants to refer to things within its containing multipart/related, but am not sure if that allows the intervening multipart/alternative.
I don't see why not. But it would depend on the implementations, which we'll have to test before recommending the structure I (theoretically :-) prefer.e