How does HT iliopsoas cause posterior pelvic tilt?
Honestly RoseofSharon, there's not enough information here. Is the tilt caused by muscle imbalance, or is the muscle imbalance caused by avoiding pain in the spine due to some condition like spinal stenosis or bone spurs? What other parts of the body are affected, and did the post tilt come on suddenly, or has the client always had this condition since you've been treating? What happens if the client tilts the hips anteriorly? Can you create a passive anterior tilt, possibly by lifting the pelvis with the client supine?
Does the client have full hip ROM? Trunk flexion/extension and rotation?
Has the client seen a chiro for possible facet dysfunction? Are there bulging discs that the posterior tilt relieves pressure?
I don't know, a slight posterior tilt is not terribly uncommon, and I wouldn't hesitate to just start treating for tight glute and vastus L & M, kind of the reverse treatment of typical lower crossed syndrome. But a "fairly extreme" posterior tilt... definitely need more information before making suggestions.
Yeah, quads will be tight, but in eccentric contraction in an effort to "anchor" the pelvis. If you start treating the eccentrically contracted muscles with typical massage techniques (MFR, stretching, TPT,) before pressure on them is relieved, you will examassageplanetate the problem. First relieve strain on quads (and iliopsoas) by lengthening the tight, facilitated muscles, then you can treat any resulting issues in the tight inhibited muscles after posture starts improving.