Just a thought - perhaps it gets so tight sometimes due to being too weak for ones Posture. Tightens up in response to overly strong muscles elsewhere or overuse?
Make it bigger and or stronger and perhaps it might not tighten up so much?
A strength reserve for posture muscles.
This is what i have been doing for clients shoulders/upper back areas.
Think of doing Innies and Outies (rotor cuff work) Your not ever gunna see these tiny muscles or be pushing heavy weights with the exercises - however, working them does Magic after just a few short wks.
Mind you - check out PNF stretching on the piraformis! Its a stretch with an active movement. Perhaps thats all that muscle needs