Actually, the first time I had heard of a use for tennis balls was in regards to maces! This was when I was still exceptionally green in terms of what I knew or thought I knew. I had read that a method for training with implements like that was to affix a target (in this case, the tennis ball…) to the end of a pole and have the mace user attempt to strike the moving target. This is not quite as fun as your other options, but for a short weapon like a mace it makes sense.