And of course, the last paragraph in the EBA states "both parties agree that NO further claims will be entered into during the course of this EBA".... or words to that effect.
REX adding this to Mr Clarkes terms of employment constitutes a breach of EBA, therefore I wouldn't think that in this case, the extra clause would be enforceable at all.
The fact that it has SINCE been added to, and approved in the EBA makes it a tad harder.... but NOT impossible.