On a brighter note, some ridiculously cool things are possible here in the future. I just found out:

• In sway, you can create a "headless" output (a fake monitor) with swaymsg create_output. You can set its resolution and position as if it were a real screen (e.g., to the right of the real monitors). Then:
• You can share that monitor with wayvnc -o HEADLESS-1. That will expose it via a VNC server listening on localhost:5900.
• Then you can make an ssh tunnel from another machine with ssh -L 5900:localhost:5900 user@remote, and open a VNC client on the other machine (e.g., wlvncc localhost; use -s to disable GPU if needed).

The result of these shenanigans is that you can use another machine as an extra monitor for the main one. It behaves just like a regular monitor for sway: I can drag a window to the other screen, and it shows up on the other computer.

(thanks https://www.reddit.com/r/swaywm/comments/gffi1h/comment/mgkhbc0/)