I have a single 6870 and use MSI Afterburner. Here's what I had to do to be able to clock the memory down to 300.
In the MSIAfterburner.cfg file the UnofficialOverclockingMode option has to be set to 2. If it is set to 1 PowerPlay (AMD's power saving feature, like Intel's SpeedStep for CPU's) is active and will not let the memory downclock from 1050 to 300. Once that is done, you will notice that PowerPlay is still working as the core clock will be 300 and the mem at 1050. As soon as you change the mem speed though, you will notice that the core clock goes to 900 and stays there, and the mem speed is whatever you set. Close/open AB as many times as needed to get to the mem clock you are looking for. Once done the core clock will be at 900 (unless you are OC'ing) and mem at whatever you selected.
At that point you can do whatever you like (run miners at 100% utilization) and the mem clocks will stay at whatever clock rate you have set. They will not jump back up to a higher clock.