I think that is an excellent idea, and more like real life. Everyone suspected Michael Owen had a release clause at Newcastle but no one knew about it until it was almost expired. Which raises another suggestion:
More clauses within the release clause. Michael's, for example, meant he could move to a Champions League-qualified team if they offered £xxxxx (15m I think). It also had a time-limit as to when it expired. At least that's what was reported. Am sure there were other conditions attached too, like if Newcastle qualified for the ECL the clause would either expire or increase.
It would help mid-table teams buy in bigger players. Not suggestion you should need a lawyer sat beside your PC while you play, but a few more clauses would be useful.
As Leeds, 1st season back in the
Prem, when a player says he isn't interested I always insert a "0" relegation release clause. Doesn't always work, but helps. Am sure the Aston Villas and Tottenhams out there would benefit from the ECL release clause.