Is it possible to pin distinct Bones of a Model to a Plane while posing? I need several poses and would like to know if whether it would be possible to pin the feet or hands of a model to a plane (as floor or wall) while animating excercises like squat or push-up ? 

If IK would be the answer: Setting an inverse kinematic constraint usually allows me to move the legs by moving a foot, but I want it the other way around. I used rigify and I'm still intimidated by the complexity of this rig, how should I setup the IK chain (without breaking it) ?
A meta-rig created by Rigify is set to forward kinematics by default. With the settings shown in the n-panel on rig-foot.ik.L/R it possible to move the body with the foot on floor by only moving the rig torso (shown in 2nd img).