I hope are reps are smart enough to not file a grievance that has no merit. First off I suspect you are talking about past practice not status quo. To try and use status quo to make a case the company can't award raises to other employees is so far beyond stupid as to be laughable.
If you mean past practice that is a term that covers grey areas in the contract. 3b4 is not a grey area. The company at one time gave Christmas bonuses. They are not in the contract so you can't as a matter of past practice demand them every year. The same applies to the shared rewards program. The raise was 1 Dec and per specific language a 3b4 review was triggered and the contract followed.
If you could somehow claim that past practice applies to areas in the contract that are plainly spelled out you run into the issue of the fact that the companies past practice has in fact been to award non contract employees a raise when a pilot TA has been reached. Contract 2012 being the most recent example.
What we need to be talking about is how to get the company back to the negotiating table because I have news for you. They aren't coming! That is where are focus needs to be.