I have repaired such damage on a NR derailleur and it worked fine afterward. You need to find a piece of metal to lay across the front plate so you don't damage it. Then place a small C clamp on the top of the bulge and across the plate. Turn the clamp lever until you have flattened out the bent rear plate; you will probably need to remove the cage to do this which is quite easy, especially of you remove the cage pin. You should also check that the parallelogram is not twisted; this is also repairable. Let me know if you need directions.
Edit: and you will need to replace the pulley wheel. The OEM run about $100 @ pair if you can find them. Soma makes a reproduction that is well made.
http://store.somafab.com/sonujowh.html