Well, if you really need this, you can sorta hack it. Open curves editor in VFB, change interpolation to linear splines, and adjust green and blue channels like in attached picture. Now you'll have purple colour for highlights clipping and blue for shadows.
P.S. you can enlarge curve editor by clicking and dragging over its edge, so that it would make easier to fine tune clipping points.
P.P.S. there is a bug, when changing channels or resetting changes, doesn't update interpolation status - i will report it as a bug.
P.P.P.S. you could request proper clipping indicators implementation in VFB, i think that would totally make sense ;] Ah crap, you already did :]