It seem's logical to me to simply have a single foot bone for a simple skeleton and have the 2 leg bones as an IK chain pointing to the foot bone. This way you have a single bone that can be moved and rotated to control the foot and the legs will sort themselves out, also if you move the hip around the feet don't inherit the rotation of the shin bone and stay planted to the floor.

This can be done in spine but you can't have the foot as a child of the rest of the leg ("The [IK] target must not be a descendant of the parent" - Spine docs) and when you get the IK "pick" pointer, you need to click on the bone in the hierarchy or it creates a null - which seems like inconsistent behavior as best, if not a bug.

So a bit of a fiddle to figure out how to set it up, but now I have a simple rig which is working exactly as desired in Spine.
However in Unity3D, I tried adding the ragdoll script and the body fell-off the feet, because they're not a child of the leg.

Is there a "don't inherit rotation" option I've not come across? Is nesting IK targets (one for position, another to control foot direction) really the only option, or maybe I could edit the script to account for the feet not being descendants of the leg.

Any ideas? What have I missed or misunderstood?
Thanks

Oh, there is a "don't inherit rotation" option. How did I miss that?! That'll do.