@nprofile1q... Thanks for all the research on this. I haven't done multi-seat stuff since before Xorg (XFree86). Now I want to again! My use case would be one small level up thought....
I use Deskflow (in the line of synergy/input leap/barrier/etc.) to use one set of inputs across a couple local devices, including sometimes ones that are also in use with their own devices. What would be amazing is if those could fire up a new seat on the remote devices instead of hijacking the existing local inputs.