I too wanted something similar - masking out windows and wanting to adjust the exterior view in equirectangular mode.
The following is a bit cumbersome, but something of a pseudo-workaround. It still doesn’t allow for live adjustments within the equirectangular projection, but it does at least make things non-destructive.
Duplicate the layers/masks before doing the Merge Down, then merge the duplicates and view the equirectangular. Then, if you realize the mask needs adjusting, you can simply delete the current layer and make the necessary adjustments back on the original mask layer.
Yes, it’s really time consuming, and not optimal. But by automating the duplication/creation process with a macro, this might at least speed it up a little.
