I wish we had something like Markdown for email--to me that would be the sweet spot between the mess HTML email has become, and plain text which sometimes does lack in its ability to communicate information.
I don't use mutt but IIRC a basic feature of both mutt and similar (pine + variants, etc.) is allowing you to define your own external editor. Therefore, there's nothing to stop you adding a wrapping pipeline to that so that, for example, first you edit normally in your preferred editor, and second an established piece of code or basic series of regex replaces your markdown with whatever you actually want. Optionally, the reverse runs before editing. This should not take longer than a few hours on the outside to get running comfortably.