Is there a reason you can't add another bone to your rig? You would be able to use the same parent point as the foot, which I think is what you want. I think it has to be linked to the most recent bone, then set the rotation point to match the foot. It will not look like there's a new bone, but there will be one.
However, I know the bone tool is highly buggy, so that could be a downside. Flash stripped out the bone tool in more recent versions (and has promised to rebuild eventually). But what you want should be feasible, at least until you start animating with it and it falls apart