The Draw Dataset plugin does something like what you want. You can start with a blank (all zero) mask or start with a mask created via 3dcalc, 3dmerge -1clust, etc, and then edit it.
You can only edit a mask (as Overlay) if it is stored at the same resolution as the Underlay. This is one of the most salient restrictions of the Draw Dataset plugin -- see the Help on this plugin for more info.
Happy Memorial Day!