Creative route nearly solved.
Turns out you can make word parse HTML or openXML inside the same AltChunk content. Docs exported to html or mhtml contain the correct formatting to do so. Not only that, but footnotes, for example, export with a particular html structure and attribute set that allow you to reimport them perfectly via AltChunk.
Maybe you were aware of this, but I just thought I’d contribute back.
My last remaining bit is getting it to accept base64 image data via this route