Did you try with the latest drivers?

Catalyst 11.6 seems to have the restriction removed (you don't even need amdoverdrivectrl)

Adapter 0 - AMD Radeon HD 6990
                            Core (MHz)    Memory (MHz)
           Current Clocks :    955           1250
             Current Peak :    955           800
  Configurable Peak Range : [500-1200]     [1250-1500]
                 GPU load :    0%

Adapter 1 - AMD Radeon HD 6990
                            Core (MHz)    Memory (MHz)
           Current Clocks :    955           1250
             Current Peak :    955           800
  Configurable Peak Range : [500-1200]     [1250-1500]
                 GPU load :    0%