Quote - Setting up joint limits, at least in the legs, may help us, but I wouldn't want to force unnatural limits.

Well real shins don't do zrot (at least mine don't). I don't think they yrot either. When it looks like those things are happening, I think it's really thigh twist that we are seeing. Besides I not suggesting forcing the limits, just setting them, so they can be turned on and off via the File menu.

Quote - Is there anything else you can think of?

in the pose file:

hip ytran
shin and thigh yrot

As well as hipHight (the parmeter), I think actual ytran value if the hip in the pose is worth investigating. bringing it up a bit might prevent the buckling. Playing with the yRotations of the shin and thigh may also help, but sounds devilishly complicated.