Interesting. I wanted to get a multiseat solution a year ago, when I used to have my office sharing a wall with the living room. I would have just used standard VGA connection and maybe an USB hub for the keyboard / mouse.
Though, considering the points some people made in this thread, the USB 2.0 might not be the right choice. I was wondering if Firewire (with DMA) or USB3.0 wouldn't have been better.
On the other hand, I would be interested in seeing that software part. Configuring (persistently) the new seat seems to be the biggest issue with the current state of multiseat, and also you need a graphics card head for each seat - that's where that onboard GPU comes in handy. Then, the only limitation becomes the length of your VGA/DVI cable....
Here's a great howto, by the way: https://wiki.archlinux.org/index.php/Xorg_multiseat