Thanks, but I found a solution by using Diablo miner. Diablo seems to use a flat OpenCL device enumeration, without a platform flag. In my case the 5970 cores are devices 5 and 6, whereas devices 1 to 4 are the cores of my two Nvidia GTX 590 cards. Combined hash rate is around 1 GHash/s.
And I can play with Quad SLI with the same system. Perfect
The only minor problem I have is that I have to disable the ATI devices in Control Panel in order to use PhysX. But I guess this can be done by a script as well.