I know that in Sculpt mode you can use Alt+M and Alt+I to clear or invert it. This is super important for a fast workflow. I tried to make my own new Mask Flood Fill shortcut under Image Paint by adding "paint.mask_flood_fill" to the identifier field but it doesn't seem to work.
Am I missing something, is there no other way to do this but manually painting away the old masked areas by hand or adding a completely new image mask?