Feature Request : Bitmap Fill behind tool

I use the bitmap layers and I can see my background art through my character line drawing. It could be nice if there was some kind of feature where I could fill in the character with white color. Right now a have to use a white Brush with paint behind and do it manually. There is is Paint unpainted feature but it only works on the vector layer. When I storyboard in Photoshop I have made an action where I make a magic wand selection in the top left corner with tolerance 0, invert the selection fill behing with white color and remove the selection. But a feature request for som fill behind feature on the bitmap layers.

I also think this would be a great feature. Would save a ton of time.