I have created a two-man squad. One 'player', one playable'. One is a higher rank than the other.
I have also created an empty humminbird.
The higher-ranking person can enter any position on the Hummingbird and its side boards. The lower-ranking person can only enter the pilot/copilot seat.
Create a crewman and a sniper. Set crewman to 'player', rank 'corporal. Set sniper to 'playable', rank 'private'
Create an empty Hummingbird.
Test the level.
The crewman will be able to sit on any position, but the sniper will only be able to take the co-pilot seat (once the pilot seat is filled).
Now go back to the editor. Swap the ranks (so crewman is now private, sniper is now corporal).
The problem is now reversed - sniper can sit anywhere, crewman can only be copilot.