Good suggestion. A fast and easy way to say sorry is also the "QQA" command.
I almost always do QQA when I hit a teammate. It's very quick once you get used to it and can even be done while you are in melee without leaving you vulnerable.
Plus it's green text!
Most people I TW, don't ctrl-m me (and I'm an xbow who ctrl-m's others all the time)... I assume its because I QQA almost every time... maybe its because people aren't ctrl-m naz1's like me.
A notification wouldn't hurt. I like the idea.
Could even add a little coding to have like Ctrl-B to make you say something like "Forgive me, (player you hits name)!" when you hit someone.