BG, we've tracked KO's in 31 tournaments to date. We've had 3 ties. I agree they are rare, but they do happen. Two of those instances were the same hand that involved 4 players, two of whom were all-in. That was easy -- we credited each player with 1 KO by crediting one with KO'ing one player and the other with KO'ing the second player. We did that only because it was faster than writing both names down twice and it had the same net effect. The other time each player got 1/2 of a KO.
In all but one tournament a year, we split bounties. If 2 players KO a player, each gets 1/2; if 3, each gets 1/3; and if 4, all get 1/4. We write them down. There's nothing wrong with using an object, but it does have the issue of splits being an issue. There are several ways to handle splits. Some give each player full credit for the KO. Others split it like we do. Neither is right or wrong; they are just two ways of handling the issue.
We have one tournament a year where there are no split bounties. Players know the rules before the game. In that one tournament, the special rules are part of the invitation, so anyone who comes is presumed to have read and agreed to those rules. The special rules are well-known to all players and reviewed before the game verbally. Thus, no one can claim it's unfair. They can claim they don't like it, but they can't claim it's unfair because it's known in advance. We've played that tournament 3 times and have yet to have a tie. It's also the one time per year we use a chip designated a bounty chip.
In the situation you describe, the first player to bet enough to have put C all in, if C calls and loses, wins the bounty. Since A's bet was enough to have put C all-in, and A was the first to make such a bet, A would win the bounty. Changing the facts slightly, if A bet 4000, then B went all-in with 6200, and C called 4500 all-in, then A called B's all-in, B would win the bounty since he was the first to call the amount of C's all-in and A's bet was not enough to have put C all-in. Since A knew the rule going in, if he wanted to collect the bounty, he should have bet enough to put C all-in. In any other tournament of the year for us, A and B would each get credit for 1/2 of a KO.