Well, i see the problem this way - the ejection dont have to force you to loose mission IE you have completede main objectives and then ejected, it is victory. you ejected in the escort mission - you go loosing branch, like you have lost the escorted ships and so on.
Code wise, it should be pretty easy - just a few sexp's, not more complicated than autopilot code - evry time key pressed etc., then script explodes the dummy. than it's just a copy paste to mission. the only REAL problem for me, is to make the dummy look like your ship, if you can choose it