rules on archer decoys are impossible to enforce. its just an accepted part of the game; not out of desire, but necessity.
Maltrease had the best archer decoy set-up i've seen - in domII - a single unit in every second square, checkerboarded, for the whole front half of his battlefield deployment
I was in awe, but i guess not ever desperate enough to ever mimic it ;p
after all, doing something extremely boring is a cost itself...