Sorry I'm not sure about that, but my guess would be that it wouldn't work. Ek makes their own backplate though.
http://www.ekwb.com/shop/ek-fc680-gtx-dcii-backplate-black.html
http://www.frozencpu.com/products/16564/ex-blc-1169/EK_ASUS_GeForce_680_GTX_DCII_VGA_Liquid_Cooling_RAM_Backplate_-_Black_CSQ_EK-FC680_GTX_DCII_Backplate_-_Black_CSQ.html?tl=c311s1631b133