I dont know how its going to work, but it might be good to base the "success" of a player on their score in a qualifying training map as well as, an overall rank.
ie, there is a 5 min training course for each asset class that teaches how to use that class. A training course for sniper, SL, MG, AT etc. which rates from 1-5 (5 being the best) you on your competence in acting as that class should. An explanation of how that class should behave, followed by a test similar to the CoD coop exercises that they had in MW2(dont flame me too hard).
This test would have to be semi-randomized to avoid a person just playing it a couple times through to get top priority. Even if it isn't randomized, the explanation at least would make new players aware of how the class should behave. Obviously this doesn't replace in game experience, but it helps to avoid too many people just taking the class for the "good" gun without knowing how it is supposed to work.
This would mean that if 2 players are contesting for the spot as SL, and both were of similar rank (within 5-8 ranks) then the player who has proved more knowledgeable/competent in the training would get priority.