Yes, it has been done... demos at least, and is currently being revamped. It does take a lot of coding knowledge to do, and even more so when you want to make a worthwhile AI. It is a difficult task, but completely possible.
Yeah... I was meaning it in the sense of adding all the things from Age of Empires and other well known RTSs. The larger majority is just variable work. But when you get to multiple teams, things get more complex. Especially making a good AI that is not cheap and is still fluid. You can do it through built in methods, but as those add up, the efficiency usually goes down.