Problem:
I want to be able to easily create multiple stances that can have arbitrary feet positions. I also don't want to create a whole bunch of custom animations for each stance. However, this means that certain actions will cause the feet to slide around.
Solution:
Since the leg chain is already locked to the Foot IK bone during the IK process, I can lock that IK bone in a location by using an anim curve to define when the foot should be "planted".
Result:
Happiness.