I hacked up something like this for Apple Notes. It’s far from perfect, but at least it lets me type Markdown-style punctuation and get the expect results. Think of it like a crummy Vim mode sort of thing.
If you want a Mac email program that renders Markdown, definitely checkout MailMate[1]. Indie developer, has already shipped a lot of updates this year.
System-wide Markdown editing would also be awesome, perhaps as a feature of the keyboard.