I own a 1951 11-48 with a full choke barrel. That gun saw a pretty steady diet of 1 1/4oz #2 steel with no measureable damage. No walnut behind the choke and no undue wear to the choke itself.
That said, bear in mind that the choice was mine and I made the choice I did because I a) did not have any real attachment to the gun, and b) I wanted to see for myself what would happen. There is really no danger in shooting steel out of your gun. About the only damage that is possible is a bulge in the barrel right behind the choke area. Now I've never seen one, but Roster claims that the bulge is purely cosmetic and will not effect the performance of the gun.
If you're still leary, consider the CIP recommendations for the use of steel loads in guns not specifically proofed for steel. For a 12ga, keep the load down to less than 1 1/8oz, the velocity to less than 1400fps (CIP actually says 1300, but that's at a shade over 8 feet, not the 3 foot SAAMI spec), the shot size to #4 or smaller and the choke no tighter than modified.
Frank