Take a look at this video - https://youtu.be/SB7T94PaNDo  Similar riser card concept.
The key is using the PLX Technologies chips that you can see in the video - http://www.mouser.com/ds/2/38/PEX8712_Product_Brief_v1.0_20Oct10-514315.pdf
It can be done, but at what cost and software will need to evolve to support all the cards on one OS, and the multitude of other reasons (like one motherboard dies and lots of hash goes down).
I already use the -wd 0 command in claymore and if I see the hash rate drop in the monitor, then I know a GPU needs some attention (overclocked too high or dead).

Some of you guys are already using a second power supply and should test (on the 12+ rigs) if the second PSU is shut off - does the miner continue hashing away, and just the GPU's attached to the turned off second power supply stop working, or does it take the whole miner down?  If not, then I think eventually we will see 4u rack enclosures with just the GPU's and power supply in each, with long USB 3 cables routed down the rack to the motherboard in the bottom 4u enclosure that has the motherboard and multiple pci-e switching cards.  To big for my budget, but something to envision.  

Edit - The card in the video is selling on Ebay for $125 US FYI - http://www.ebay.com/itm/142479750737