You don't need an MMU-capable CPU to render remote graphics. You don't even need much more RAM than a local framebuffer, which for low resolutions/color depths is very little RAM.
Proving this point, there are VNC client implementations that can run on MS-DOS machines.
msgodel
You probably do need it to run Xorg though. I'm unaware of an X server that can run on DOS.
Proving this point, there are VNC client implementations that can run on MS-DOS machines.