At work, people send email asking for help. It's our practice to respond to the email letting the sender know we are working on it even if we don't have the answer right away. This seemed like a perfect application for AppleScript. The first applescript example I found that seemed relevant was to Extract the name and email from selected messages. This showed me how to operate on a list of messages, but I wanted to compose a reply. I was wondering how people knew what you could script with each application when I found out that Script Editor has a Dictionary Browser. Whoa! Way cool. Now I could compose a reply. All I needed to do was insert my text and I was done. Wow, this was really easy. Naturally, I was wrong. I simply couldn't find an example of inserting text into the reply. If I tried concatenating my text with the text of the reply, then the reply lost all of its formatting (e.g. the original text was no longer quoted). The only reference I could find was to use call method, but Script Editor would give me a syntax error every time. I was really pulling my hair out now. I seemed so close. I looked at the AppleScript Terminology Reference, but that didn't provide any clues. I looked at the Data Dictionary again and I noticed the 'make new' command. Maybe I could create a new text object and concatenate it with the quoted reply. That's when I found a script to Send an email with an attachment. It had the very interesting make new attachment. Maybe I could create a new text object the same way. Bingo! using terms from application "Mail"
    on perform mail action with messages selectedMessages
        set AppleScript's text item delimiters to ""
        repeat with eachMessage in selectedMessages
            tell application "Mail"
                set theReplyEmail to reply eachMessage with opening window and reply to all
                make new text at before the first word of the first paragraph of content of theReplyEmail with data "Working on it." & return & return & "Drew" & return & return
                send theReplyEmail
            end tell
        end repeat
    end perform mail action with messages
end using terms from Somewhere in this thread is a tip that explains how to use System Preferences-> Keyboard and Mouse to set up a keyboard shortcut for a script. You need to restart Mail.app for it to take effect.