An even worse hack, that unfortunately has worked really well for me in the past: generate a simple HTML file (em, b, u, ul/ol, etc.), rename it to .doc, and Word will open it without appearing to notice or comment. This saved my butt once when I discovered mid-project that the Textile library I was using couldn't generate RTF... I felt more than a little bit dirty, but it worked without a hitch.

> .eml and .mht are actually the same format.

.mht uses MIME/Base64 to store all of the HTML and external files together, but IIRC it's not exactly the same as an email because the headers are different/missing (i.e. I wouldn't make sense to have a To:/Reply-To:/Cc:/etc header)