Make sure one of the machines is Derby Day; it was in the “D”/Juniors/Seniors/Women’s bank at PAPA recently. But seriously, it would be fun if you had horse-themed machines in the event. Hot Tip; Winner; Cowpoke / Buckaroo; if you wanted a newer game, you could include Mustang since it both involves racing and is named for a horse.
As for your format, I like it. The one downside is that the odds tiering may be uneven. If one or two horses have really good odds, but the next three or four are so-so, then which machine someone gets assigned to makes a lot of difference. For instance, if the first 5 picks are 5-2, 3-1, 8-1, 10-1 and 12-1, those first two have a big edge.