By default, the game uses a square "edge" texture for the cooldown swipe. This is basically just a line that leads/follows the colored swipe and is also used (I think) for the charge cooldowns. Hopefully this is what we think it is so I don't have to hack to make circular swipe edges.
@Myrroddin: I imagine it's unique to the Cooldown frame, as is SetSwipeColor, SetSwipTexture, SetEdgeTexture, etc. But action buttons, buffs, etc, typically have a cooldown region inherently, IIRC.