I don't really care which side I'm placed on. Perhaps some of us can be designated floaters to balance things out?
There are supposed to be detailed orders for each side a couple days before the event. Since only registered squads are allowed, each member should have access to their orders well before the event.
To reassign folks aware of the orders for one side to the other will invite an elevated level of paranoia that mission details were 'leaked'.
No matter the integrity of the players involved.
From a purely logical approach, at launch time, the CM should declare how many of the side with excessive numbers will need to sit the frame out.
This, of course, cannot work, nor would it be a good idea to even try it. Those told they they couldn't fly would likely never return. Really bad idea.
The only thing I can suggest would be that the imbalance numbers be assigned to an aircraft that is inferior to the the opponents. So, if the allied were 18 over in numbers, the CM would enable some plane that is lower value than the A6M2 and insist the allied CiC assign 18 players to fly those planes... and, you know what, that might be tough in this setup. I'd say P40c, but even that is faster than the a6m2.. I think.
To summarize, there is really no way to fix the numbers at the last minute. Yeah, it sux. I just don't see how it can be fixed. It's up to the player base to show up.
My $.02