Wow.
It's still available. I unfortunately don't know how well 3D would do on a link smaller than x16. I think there have been studies on it to show that the gaming performance hit would be nil, but rendering and other such workloads would take a major hit.
This is the most powerful SHB on the market that I know of:
http://www.trentonsystems.com/products/single-board-computers/picmg-13/bxt7059It supports up to two 8-core Intel Xeon E5-2448L processors (16 physical and 32 virtual cores with HT), and up to 96GB of RAM if you use 16GB modules in all 6 slots.
If you will be processing video, you will want this backplane instead:
http://www.trentonsystems.com/products/backplanes/picmg-13-backplanes/video-processing-gpu-backplane-bpg8032It has x16 links on all slots.
As for how many cards you can actually install and use... you would be on your own there. It requires virtualization and/or driver tricks that I have no idea how to do (that's mainly why this project flopped), and possibly BIOS mods depending on how you are trying to make it work.
If you want it, let me know.
All things considered, there's a lot of limiting factors to the idea I've got as far as this backplane goes, but with that SHB, while the chips on it would be good for a parallelizable workload, they'd likely be terrible for gaming

I was doing some math... Without an AMD option, even if I could pack the machine with the appropriate Intel chips, it'd likely be more than 3-4x the cost of putting together multiple boxes, even considering duplicated hardware such as cases, power supplies, and so on. I naturally expect a premium for enterprise hardware, but at some point.... I'm sure you get th idea

Part of me really wants to see the sight of 15 or so monitors plugged into a noisy metal behemoth.

Shame I didn't catch this thread back in February. Even if VTd passthrough weren't an option, I probably could have helped you with using Xen PV passthrough to make it all work, and that would have been a real pleasure to see!
Oh well. If you want to take a stab at making it work again, drop me a line. I could at least point you in the right direction

Also, thanks for the quick reply!