I have had similar experiences with cast wheels and straightened them myself. However, to repair the damage I did not use a hammer, rubber, lead filled plastic or otherwise. I removed the tire from the rim and used my press and hardwood blocks to both support the lip of the rim adjacent to the dent and one between the dent and the ram. Worked like a charm with no evidence of harm remaining when done. Since you probably don't own a press, you can probably employ someone to do it for a nominal fee. I would never buy a new one to replace a rim with only cosmetic damage. If the damage was more severe, then a replacement would be in order.
If you do decide to use a hammer, I suggest that you remove the tire and place it on something quite solid that is soft enough give when you hit it. I would put it on three equally spaced 4 x 4's and then cut two pieces of hardwood to fit snugly between the beads adjacent to the dent and then whack the dent. These two supports are crucial because you will do more harm than good without them. You will need a hammer of substantial weight and it must be soft enough to prevent denting the rim. Again, a piece of wood held in place by another person would be a good buffer between the dent and the hammer. You will also be amazed at how hard you will need swing the hammer to get the rim to bend.
By the way, what was the tire's air pressure when this happened?
