I guess some of the Danville regulars may chime in about basic standards (which do vary depending on which club is hosting the event that weekend).
As you've rightly pointed out, placing it on the front turret is going to raise some eyebrows, but placing it that high up on the second turret is going to hurt your competitiveness as you're essentially showing your apple over ridgelines that other tanks and their apples can stay below.
If the umpire allows it you might actually end up having a two-player tank (a driver/gunner and a gunner) where each turret has an apple and needs to be taken out before the tank is knocked out of battle. Kind of making up for the two IR emitters you could probably run...
It could actually be a very interesting and fun tactical problem.