modifying angular form (new mailing)
What about modifying the option list itself? So pick a hook that fires whenever the new mailing page is loaded (hook pageRun or something) and use the api to check if the current user's email is an option in that list. If not, add it

That gets you halfway there... not sure about setting it as the default. I suppose if you made it the default every time the hook fires, then it would temporarily be the default for whoever is composing a mailing.