Well, I have not suggestions for the ENY lock out because I don't like the system, but I have some alternatives.
1) Perk every plane, but have a minimum number of players that have to be on a side and total number for the arena before it kicks in. For instance, the P-51D would have a base perk cost of, say 5, but it would be 0 unless there were at least 150 +players on and a side had a 33% numbers advantage (just an example). Before that point, the 51 would be perk free, but above that point it would cost. As the numbers imbalance increased, so would the perk cost. Say, there were 300 on and the side in question was at a 40% advantage, the Pony would be 10 perkies.
I know the number of perkies doesn't seem a lot, but it is something and over time would add up. Also, we have the perky system anyway, why not use it instead of creating something new.
2) Modified the current perk award system so that perkies award are much more drastically effected when your side has a numbers advantage.
3) Award perkies to people switching sides (from a side with numbers advantage to one at a disadvantage). The perkies awarded would be based on the difference in the side they come from and the side they go too.
4) Time delay between flights. If you crash, get shot down or in any way end up in the tower, make the player from a side with a numbers advantage have a delay before the can fly again. For instance, have a base 5 minute delay that would kick in when there was a 20% advantage and have it go up with each percent increase in the advantage.
5) Perk everything except the ENY 20+ planes (of course, the numer could be different). That way you will always have to pay for the low ENY rides whether there is a balance issue or not.
6) Regularly rotate the side that new players start as (this may already be happening, but I am not sure). Do this on a weekly basis when the map is changed.
Now, personally, I don't think there needs to be anything to "force" a balance. Things like that will even themselves out given enough time. Short of a "rolling planeset" or nation (alliance) specific model for your available planes, I don't like anything that limits what you choose to fly (with the exception of the current perk point model).
Just my ramblings...