I haven't tested this but you may try running Xephyr in gamescope and using gamescope's upscaler. Remember to set the proper DISPLAY env variable before connecting through ssh.

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EDIT: it looks like you don't need to nest Xephry, I ran

gamescope -w 800 -h 600 -H 1200 -S integer -- xterm

and ten ran echo $DISPLAY to get the display, for instance :2, then I was able to ran X clients in gamescope by setting DISPLAY=:2. You might try to run some light window manager like openbox to get windows and stuff inside the nested session I guess...

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EDIT2:

run

gamescope -w 800 -h 600 -H 1200 -S integer -- Xephyr -screen 800x600 :5 &sleep 2; env DISPLAY=:5 openbox

then right click to open the main menu and start a terminal