I don't see why your idea wouldn't work. If I'm not mistaken, many of the mono-, bi- and tripods sold as shooting accessories have a standard camera mount that the "Y" shaped forearm rest part screws into, so you might be able to buy just the gun rest and put in on the tripod. If not, your idea of welding/bending a piece of steel and screwing that on would work, too.

To cover the metal, you could use moleskin. Bowhunters use the stuff to cover the cutout portion of the riser and arrow rest to prevent noise when they draw the bow. The stuff has a soft, velvet-like, texture on one side and adhesive on the other. Wrap the rest part with moleskin and you shouldn't damage the finish on the gun.

As for the rebar target idea... why not just build a target frame out of wood? There's always a decent chance of a ricochet if the bullet hits metal, particularly angled metal. Would the bullet come back at you? Maybe, but probably not. Would the bullet always land in the proper and safe impact area? Maybe... but maybe not. I don't know about you, but rifle bullets flying in random directions isn't exactly my idea of safe. I've had good luck with wood, and you can also try using PVC plumbling. It can be assembled easily with just glue, and is cheap.

Mike