Maybe you could have 'zone' tags - yellow for rows 1-6, red for 6-12, blue for 13-18 etc. Colour the inside of the locker lids appropriately. Everyone gets two coloured tags, all items in an overhead must have the correctly coloured tag but small items can go in provided there is room. If one locker zone ends up overfilled the surplus can be moved where there is space. Admittedly that isn't the same as CE only tags but it would solve the problem all the way down the plane - and someone who knows they will be able to stow their bag above their seat in row 4 probably isn't going to bother shoving it in over row 1.