I think “applying CSS styling to Email” is a much narrower feature than full blown RTL support. I think you can just discuss here, before coding, outline your plan so we can make sure it looks sane.
At a top level I would simply add support for an “email.css” (containing default styling) that can be overridden. The tricky thing is taking the css file and applying it inline, but there are a few gems that do this which you could leverage.