Not a fan in the least of both parties taking damage. If some bozo friendly wants to spray at me for no reason, I don't want to suffer. If that was the case I would have to worry about Green targets more then Red ones.
Your second idea though is interesting, a gun lock for a certain amount of time as a penalty for shooting a friendly. I like it, it's a bit a more user-friendly. Not sure how well it'll pan out though, your gonna get a lot of responses saying how you need to be more aware of the 500mph friendly who just flew through your bullet stream to try for an easy kill on your set. Although I'm all for SA, sometimes it can be out of your hands.
P.S. This should probably be in the Wishlist forums, I'm sure Skuzzy will get around to it.