Pretty much is a laptop running xf86-video-modesetting driver, with my server, an xterm + metacity. Plug in a USB displaylink device, with a kernel drm driver I wrote for it. (Sneaky xrefresh in the background). and the USB device displays the xterm and metacity.
So what actually happens?
The X server at the start had loaded drivers using udev, and the a new driver ABI. It exports one X protocol screen and plugs an internal DrvScreen into it.
When the hotplug happens, the server inits another DrvScreen for the new device, and plugs it into the single protocol screen. It also copies all the driver level pixmaps/pictures/GCs into the new driver. The single protocol screen at the bottom layer multiplexes across the plugged in drvscreens.
This is like Xinerama pushed down a lot further in the stack, so instead of doing iterations at the protocol level, we do it down at the acceleration level. Also I have randr code hooked up so all the outputs appear no matter what GPU they are from.
This isn't exactly what I want for USB hotplug, ideally we'd use the main GPU to render stuff and only scanout using the USB device, but this is step one. I also need the ability to add/remove drvscreens and all the associated data in order to support dynamic GPU switching.
The real solution is a still a long ways off, but this is just a small light in a long tunnel, I've been hacking on this on/off for over a year now, so its nice to see something visible for the first time.