Anthony, whilst the Quadratus Lumborum QL is a very strong muscle and does exactly what you state i have never found that directly stretching has any long term effect unless other factors are also addressed. I guess this was already discovered by the field of muscles energy technique MET although Yoga knew this a very long time ago.
In Yoga the QL is not stretched in isolation. It is part of a more hilistic approach of stetching the movement range from the leg, through the hip and into the lumbar spine. A Yoga position/stretch doing this would be Trigonasana or the Triangle Pose. The problem is you would need to explore Yoga to find a teacher who can get you to correctly apply this Pose.This is rare in my experience but worth the journey if one is really interested in the long term health and functionality of their body.
In MET, to correct pelvic imbalances, which seems to be one of the current topics in this thread the QL release is well down the list. In order of gaining a greater pelvic balance a MET practitioner would start with the most important factors and move down the list. The most important factors create the biggest change in the pelvic imbalance whilst lesser factors like the QL provide less change. The most important is the gluteal release -mostly the dominant side gluteus medium and minimus. Second and mostly in compensatory counter-balance to the gluteals is the TFL (tensor fascia latae). This dual release is the initial starting point of MET adjustment and does the most to correct the hip/pelvic imbalance. Following this adductor release, psoas release and the QL release further refine the correction of this imbalance. More advanced practitoners also take into account ankle imbalances and shoulder imbalances. After all the body is a connected whole and when one bit goes out the rest of the body adjusts.