What you’re asking for feels like the antithesis of each workspace being assigned to a virtual screen, which says “these are physically disjoint spaces”. You seem to be seeking a move command that’s willing to violate this abstraction, penetrating workspace boundaries. (Or if not, I’m not sure what you could mean; floating windows are the only other case I can think of that could be affected in any way.) The only principled way I can see it working would be if you defined another nesting level above workspaces, so that screens were rigid splits on the greater part. This could probably work, but it doesn’t feel right to me. Certainly I never had the slightest desire for anything like this when I used two or mostly three monitors. I treated one as primary working space, one as secondary (mostly for communications), and one for things I might want to keep an eye on in the background from time to time. Occasionally I’d want to switch a window between the first two screens, but I would always want to use a workspace-shifting command for that; never (not once) would I have desired a move command. Very seldom indeed would windows shift to or from the third screen—they’d start there, live out their neglected lives there, and perish there at my pleasure.

I’m interested to hear what you have in mind.