It should be possible to enter enemy Multi-Purpose vehicles (Gas Tigers and Humvees) and sit in them. This could be done if the vehicles is below a certain Health treshold.
So for instance. To enter an enemy APC, or enter a Tank the vehicle must be below 50%, after which it takes like 4 seconds to enter. Reduced to 2 seconds if below 25% for example. Same for Gas Tigers and Humvees or Boats.
Both Helicopters would have the usual enter time, this would reward pilots who use their camera below (to retract the rope to make the enemy fall) and would make awesome situations where someone jumps onto a rope of an enemy aircraft and then enters the heli to kill the pilot (or get killed by the other crew inside).