Thanks @JohnAtl. I know the regular keyboard shortcuts; I was looking for something to automate the formatting process. My latest idea is to take the original message into a markdown editor, then make a custom CSS file to reinterpret double-asterisks as “bold, indented, blue, beginning with right angle brackets” etc. That way I just mark my comments with **asterisks** and it always comes out rendered in the special style. Then output everything through Marked 2 as RTF, then copy and paste into the email client. A lot of steps, but maybe Keyboard Maestro can trim it down a bit.
So yes, more razzle-dazzle. Partly because I want to make the process easier and save time, partly because I’m a student of Mac automation and it’s becoming an obsession. By the way, I spent all day yesterday (on and off, of course) camped out on this thread you pinned. Really got me thinking. Great discussion, thanks for making it a wiki page. 