[Klemm]

These aircraft most often had bullit proof side glass plates as well.

Only the Sturmbock variants. And there were not many of them. And such a small armor plate on the side wouldn't bring much in terms of protection and would also obstruct the view. But if the modified aircraft could be added (not replace the original.), then this would be a nice addition.
But the in-cockpit view would have to be modified as well. Don't know exactly, but this could turn out difficult.