perhaps not quite the same, but bspwm has the concept of "layers", and the ability to set borders per window -- so all you would have to do is move xclock window to an above layer, remove the borders, and use something like xdotool to place it in a dock-looking location.

To elaborate on layers -- there are 3, below, above, and normal. Each layer has a notion of tiled, floating, fullscreen windows. Windows on the above layer always show above windows on the normal layer, regardless of state.