the keg barrel could work. isn't made of aluminum? If you fill with water, I'd be worried about the hole you made enlarging and eventually rip. To be honest, I have no idea how thick the metal is. if it was filled with concrete, then nothing to worry about. IF you go the concrete route, think about how you can make it a bit more useful. do you want to add a plastic tube in center for holding garden tools? Do you need to tow trailers often ? then add a flat bar. Whatever you do- make sure you rig up something for the top link bar to attach to, otherwise the keg is gonna swing back and forth alot and hit something.
Really- I have fluid in my rear tires for ballast and they make a world of a difference then without on my BX. I used BX for 2 years without fluid and then added them and I love it. never again without it. If your tractor is ALREADY at the dealer, call them to see if they can add rimgaurd or windsheild fluid. I wouldn't add calcuim cloride since its on a homeowners lot and most people take pride in thier lawns. if you get a leak in rear tire, you will get a bald spot. That's why I prefer rimgaurd. won't hurt anything.
It won't hurt anything for making a concrete keg ballast. at the most you will only lose money on the few bags of cemet and its not expensive. When I was making stuff for my 3 pt, I used scrap 1/2' galvanized water pipe and cut to length. it's just about perfect for 7/8 OD diameter.