Well, for me, my healing is primarily AOE, which you will use a lot as a priest healer. So, I don't know if you would even use mouse over for that.

You could still use a tank as the leader and mouseover to heal. Just set up your hotkeys on your tank UI so that you click them with a mouse and heal. So, you fool yourself into thinking your the healer but your not =) There are a lot of add-ons that can do this.