There is some support for this in the new email policies.  Take a look at the new examples in the 3.4 email documentation in particular.

The additional thing I would like to see is support in the content manager that recognizes multipart/related and makes it easy to manage one.  I have to find time to look at it again myself before I can provide my ideas of what the API should be.  Maybe you could take a look at the docs and examples and how the contentmanager works, and see if you have thoughts or questions.