have heard it said that Pharo already chose to be against God, so the act of God harding his heart was not changing is will but letting him continue to act against God as previously chosen.
that being one answer
i think to more properly answer the question one would have to engage in the question of what are we actually talking about when we say, free will. in no way will i ever try to tackle that question, but it seems to be that there may be something to saying that God violated pharos "free Will" and made it impossible to choose anything but to be against God and the Israelites because his heart was hardened. with that in consideration,
The Bible does not tell us of another time that God has intervened in this way, or that it is something God does. But if God did what would it change for us ? I don't think it should change anything, except maybe increase our realization that God is God and we are not.