This is something I could get behind. $100 is nice, but the t-shirt or free sub is probably a more reasonable reward.
The only way to be sure in fighter mode is for them to only fly these planes in fighter while all other planes are in attack. That is the only way to verify in fighter mode/fighter rank. This would assume you are using fighter rank to determine who did the best.
Also, are you assuming this for each plane or who ever did the best out of all the planes combined? Probably better to use the combined method as you'll be handing out a lot of free subs....
There are other factors like, is there going to be a sign up for this? Or how do you know who is participating?
Is scoring going to be based on most kills, fighter rank, or K/D?
With K/D and/or most kills, you don't have to only fly one plane in fighter mode, you could check the stats and see there. Though this may take some time to gather the information for those planes and determine the winner, especially if multiple pilots fly multiple planes.
Fighter rank would probably be the easiest approach, however if "hypothetically" no players made it in the top 50, you wouldn't exactly know who won.
While this idea is cool and fun in theory, I doubt many would participate and only like 10% of the current player base go on the BBS, so it would be tough to get a big group of players to participate as it probably wouldn't be able to be marketed in the MA to get players.