You can parent (not parent constraint, so you can adjust it later) a locator to the bone you skinned the saddle to, then parent constraint the IK foot to the new locator. Good luck!
Here's an issue im having though, cannot parent or constraint the IK foot onto the saddle...I think it's something to do with the fact that the saddle is skinned to bones? I made it simply by copying some faces and extruding them out: When I constraint it or parent, it just doesn't follow the movement.