The NFLPA reached agreement with the NFL on limiting rookie earnings to shift more money to other players, I don't know why they couldn't do the same with an individual player cap. I think they would balk at specifying a position since it would open the door to one position having a ceiling that gets surpassed by a player at another position.
If they could negotiate for increases in salary minimums (or benefits) for the other 52 players on the roster to keep the owners from pocketing the savings, it could work. They could also push for an increase in minimum cash spending to accomplish the same goal.