I’d have to agree with Kelemelan here. While on the surface it seems like a simple idea, adding a new button to the UI that’s self-explanatory and won’t get pressed more by an accident is tricky, as we already have the interface kinda overcrowded. Adding this functionality as a hotkey-only would probably make this invisible for most of the users, and unusable for the ones on mobile.
The original idea behind the Random button was to give you an initial idea to work on, and not the other way around. When you only want to randomize a single layer, you can just as much select items from that layer randomly and see which works best.
I’ll think a bit more about if/how this could be implemented, but I’m not sure there’ll be a way to do this in a meaningful way. I know this is not the answer you were looking for, but for now, I hope this helps. 🙂