The problem with magnets is not that they can interfere with the radio modems, but that a changing magnetic field will create a current flow through wire I don't know how much current this would induce, but it could possibly be enough to mess with the pwm signals or other electronics of other robots. You may have to be careful to avoid this if you use a powerful magnet.
I found
this website which has one magnet with 115 lbs. of holding force while weighing less than a pound for about $130. With a few of these, the hard part about hanging on the bar would be finding a way to release the robot at the end of the match.