My idea (which I haven't worked out in detail) is to add either a new content manager that extends raw_data_manager, or just add to raw_data_manager, a handler for 'multipart/related'.  The set handler would handle a value of type 'dict', and expect it to contain a mapping from content-ids to...something.  What the 'something' is is the part I'm not clear on yet.  Trivially it could be MIMEParts, but that may not be the most convenient API.  Or perhaps it *should* be MIMEParts, with a helper utility to make building the dict easy.  The 'get' handler would return a dict in the same format, whatever we decide that format should be.