It would require what I imagine to be a great deal of new engine coding.
I have a bit of a hard time believing this. I don't know a thing about engine coding and very little about coding in general, but this just seems way too simple to require much - if any - new code to be written.
At the present moment, we actually have this functionality in the game, albeit in an improvised manner. Simply order your ship towards the direction you want it facing and immediately order it to stop. At worst, the ship will move about two units forward, but in just about every case, it will turn towards that direction after stopping. Or just turn, considering it might not even move when you attempt this.
With how simple this method is and how effective it is, I really doubt that the engine would need any new coding. And if it did, most likely an almost trivial amount.
Of course, I'm all in favor of having this added in. What I'd like to see is an implementation like the one in Dawn of War II; holding your mouse button down on empty space allows you to order the selected unit(s) to face in a certain direction when they arrive by dragging your mouse around.