It already done, is called Hydralogix.
It came in several comercial MOBOs, Asus ROG came in my mind but there are several others.
"...3rd party graphics differ from the scaling gains due to Crossfire/SLI?"
Sandy bridge is internaly located, PCI-E need to be adressed therefore no especial chip is needed they do not conflict.
Else to switch beteewn the PCI-E cards a multiplexor chip is needed to control the traffic.
Based on PCI-E multiplexors, it can not match the speed on native solutions but for consumers like me who have good but older cards is like mana from heaven.