Force non-black display for unrendered areas in VFB?

[SonyBoy]

Is there any method for forcing the VFB to display a color other than black for unrendered areas?

I'm doing a lot of high-rez stills that have a lot of empty pixels and want to be able to see which buckets of empty space still need picking up. Currently they are black both pre and post render.

[fixeighted]

Only a late-night thought and maybe not ideal, but wouldn't adding a background layer of x colour work?