Arena country populations are generally not proportional in local fights so this could have the opposite effect from what you intend. I realize this is more true in LW than MW but it's still a consideration. I don't believe that local ENY adjustments are practical.
Maybe I'm not being clear ... I'm proposing that the ENY adjustment (in LW anyway) be based on the ATTACKERS VS DEFENDERS at a specific base ... and that the number of troops required to TAKE the base, and the amount of time that Guns, Hangers, etc stay down, be adjusted according to that ratio ... Giving the defenders a CHANCE of being successfull. I suppose you'd have to set a radius (from the map room?) inside which the count would be made.