Would it be possible in a SINGLE PLAYER level to make it so If you're driving a pelican, pressing q forces all the marines/spartans/whatever to get out?

You could make it into a door and select melee to open.

But does melee work in vehicles? And besides, I think you're in the wrong thread. I mean press *button* and all the people in the vehicle get forced out, as if you're saying "Bail out"

yes this is possible. in a script. if you have a certain encounter set to be in the pelican, then have the script check your flashlight state or something like that, run the command "vehicle_unload_magic"
basically, that means if you press q, the specified encounter in the specified vehicle(pelican) will get out of the vehicle. if you can get that working it would be pretty cool

Sure, easily. Just test for the flashlight state of a pelican, and when it's on, unload all the passenger seats of the Pelican. Here's a quick script:

(script continuous dump_marines
	(if
		(= (unit_get_current_flashlight_state pelican_name) 1)
		(begin
			(vehicle_unload pelican_name "passengers")
			(unit_set_desired_flashlight_state pelican_name 0)
		)
	)
)

I'm not sure if it's "passenger" or "passengers", and I'm not sure if all of the rider seats have that marker name, but it should work.

did you make sure all of those commands are valid? i dont remember seeing vehicle unload, but either way good job on that.
/me saves