I think a one click/key shortcut for this sequence of action would satisfy this.
- Add a blank global selection mask
- select the newly added global selection mask layer
- Switch the brush to anti aliased and colour to black
Now user paints the masks, presses the short-cut again
- The user is on the original layer that they were prior to entering the mask editing. with the selection based on the paint active.
Or we could also ask to bring back the selection brush that was in krita prior to 2.9 (How to select areas with the paintbrush in Krita 2.9 | Krita)
Also we have had the similar feature request here - Feature request for a freehand brush selection tool I think we should merge these two.