Trebuchets are relatively easy to assemble, made of readily available wood, and more than capable of yeeting the fattest of fat cats. They are also legal to own and require no permits.
Personally, I’d love to turn it into a sort of Punkin’ Chunkin’ type event. Have different categories for accuracy and distance, then a separate division for non-traditional “cannon” type launchers.
Lastly, separate the launchers based on how the launcher is powered; Cannons may be pneumatic, but there should be a separate category for human powered launchers. Just like in Punkin Chunkin, the person has a limited amount of time to “charge” the launcher, via some mechanical means. Treadmill, hamster wheel, rowing, etc are all acceptable, as long as the launcher is entirely powered by the one person.
And just like in Punkin Chunkin, the shots only count if the billionaire is intact after launching; If they’re completely turned to mush while in the air, the shot is disqualified. Bonus points for keeping them conscious and screaming/flailing in the air.