The interesting question is that can he run X server locally in 4k 120hz mode.
The next interesting question is that can someone hack drivers to give ethernet over HDMI cable. Or atleast forward X commands from application running on desktop to monitor.
From what I've read I don't think it's possible to get 120 Hz @ 4k due to bandwidth limitation with HDMI 1.4. I read that with certain monitors and recent NVIDIA drivers you can run the monitor in Y:U:V 4:2:0 mode to reduce the bandwidth and then get 60 Hz but the color quality would be poorer (assuming it even works). All the things you propose would be very interesting, but I don't have the time or expertise right now to do this. I got one of these monitors that was shutting down randomly and this work was primarily motivated by debugging what was wrong hoping I could fix it. I ended up sending it back before I started this work hacking the firmware.
The whole point of the exercise was to get rid of bandwidth requirements from the cable. Its running that locally on local X server. X server commands do not require moving all the pixel data, they do not all that redundant data like logos and pictures, the text you see isn't at X protocol level group of pixels its bytes to a font. You move the icon once through the cable and the server will hold to it. That is beauty of X server being capable of running remote apps. From the server point of view your local computer would become remote computer running the application and the monitor would become the computer, handling input and running local apps which pretty much only consist the server that handles the graphics.
wonderful someones hacking on the Seiki 4k since the release a few months ago.