Ooooh, that's a toughy if you have the feet parented to the shin bones.
I don't typically use IK that often so someone else may know a better work around, but off the top of my head I'd say maybe create a second pair of feet that aren't parented to the shins(parented to a static bone preferably). You'd make those extra feet invisible when handling any other animations but for the idle ones(or anytime you want your feet completely static) you can make them visible and the shin parented feet invisible.
I hope someone more knowledgable comes along, this feels kinda cludgey to me.