I welded a bit to my pneumatic hammer with a screw driver handle welded to the side. This lets me vibrate the rust out and turn the bit with the screwdriver handle at the same time. I actually made one for phillips and straight bits also.
I made one of those and it works like a champ. I just ground the end of the hammer tool bit into a 3/8" square so that I could use the 3/8" drive clutch head bits on it.
The beauty is that this tool does not apply any torque on the fastener. It only applies the hammering force to loosen up the rust. The torque to turn the fastener is applied by your hand on the welded-on handle on the tool bit.
