Weird. AMDOverdriveCtrl works fine for my 6870 under Ubuntu. Can't see why it wouldn't work for you. Are you running the latest ATI video driver and SDK 2.4?
Can you confirm that you can able to REALLY under clock memory of your SPECIAL 6870 card in Ubuntu with catalyst 11.6 & AMD APP 2.4 or whatever version of catalyst you are using & APP?