Instead of killshooter, how about one of these alternatives (in all of the below, the friendly target take no damage):
1) When you hit a friendly, the damage value of your shot is subtracted from your perk points for your ride type (fighter/bomber/vehicle). When your perk points go to zero, any additional damage to friendlies is accumulated as the number of minutes that have to elapse before you are allowed to launch in a plane or vehicle again (you can still join a multi-place plane as a gunner). An arena-setting scale factor would be used to adjust how fast friendly-fragging eats your perks.
2) I don't know if the host has enough data to be able to tell which 'burst' of fire a round came from, but if it's possible, then when you hit a friendly, all damage inflicted by you on enemy aircraft/vehicles for five seconds on either side of hitting the friendly will be credited as having been inflicted by the person you hit for determining kill credit. This would discourage shooting past people to get kills, because if you tagged the person you were shooting past with even one round, all the damage you do would go to their credit for getting the kill.