Theoretically, yes. But then you should probably plug in the mobo power connectors to boost the pci power too.  Then you also run into the inferior quality of long ribbon risers and potentially risk a short.  I personally would go with the usb ones since then the power won't be dependent on the pci slots and you won't have to worry about overloading anything internal in the motherboard.

I have used powered ribbon risers though, and as long as you inspect the solder work beforehand and are very gentle with them whenever you're doing any work on the rig, they should work well.  Even in that case you have to be careful though. Any wrong moves or a sketchy solder joint could send power back down your ribbon to the board.

In the end, I think they all will work fine if you don't get any bad parts and have a powerful power supply, but there is less risk in my mind when keeping the power draw relationship between the gpu and psu as much as possible and avoiding overloading the mobo by powering them all from there.  Saw one too many pics of burned boards near the 24pin i guess