That's a cool idea. I can see a few problems when going from mask to slice, because of how the outlines work, but that's for us to solve. I'll put in a feature request for you. No promises on if and when though!
Btw, keep in mind you can set slices to have a black background, which masks out everything behind them, effectively making them a mask when not playing content.
https://resolume.com/manual/en/r5/outpu ... n#black_bg