I think my suggestions stand.
I haven't used the OpenXML altChunk stuff myself, but I'd guess that if you are using an HTML chunk, you can style it via CSS. So that means including the CSS in a <style> tag in the altChunk (or apparently, using an MHT file, an including a .css in it).
So if you are including the CSS in an *HTML* <style> tag in the altChunk, the question becomes, how do you create that from an existing Word style?
And my answer to that is in my previous post
May I ask why you are using altChunk? If you need to change how the HTML is styled, might it not be easier to convert your HTML to OpenXML?
cheers .. Jason