You need to save a new CCC profile.
Then you navigate to C:\Users\YourUserName\AppData\Local\ATI\ACE\Profiles\ and edit the file ProfileName.xml
You will see something like this:
<Property name="Want_0" value="xxx" />
<Property name="Want_1" value="xxx" />
<Property name="Want_2" value="xxx" />
Change each value to desired MHz*100 (30000 for 300 MHz). The 3 values are different clock targets depending on load, so changing just the 3rd one should work too, but I'm not sure if it can be lower than the other 2, so I change all of them to be sure
Then just save the file and apply your profile in CCC. The minimum clock should now be 300 and you can move the slider down.
Note: If you're not on windows 7/vista, the profile will be in a different location.