This would give priority to the highest scoring players first, via on screen prompt. "would you like to enter your behemoth" press "button 1" yes "button 2" no.
Then with a little magic teleport you to appropriate seating, off the battlefield.
If the behemoth still has empty seats when reaching the border of the map, then other players should be allowed to jump in.
This could give the key players on the losing side an even better chance of a comeback.
When teams purposely fall behind to get the behemoth, this will make their logic obsolete. For you have to be a player actually trying to win, getting points, gaining first priority.