Oops, my bad. Either of what I suggested causes "Attack [friendly]" to come up in Alpha wing's orders when you target them, but they don't actually follow it through. That doesn't seem right, to me.
Another probably nonsense idea - change the friendly's iff to hostile, and then it's iff colour to green. Would that work?
EDIT: To answer my own question, yes, I tested this briefly and it does work. This does come with the problem that the "friendly" target will fight back unless ordered not to. It will also be treated as an ally of any other hostiles (you can overcome this by using the neutral iff rather than hostile) and will trigger battle music cues (you may or may not mind this). You'll also need to switch its iff back to friendly afterwards.
And just to clarify, using this method wouldn't require any changes to iff_defs.tbl.