Do your overflows have a single drilled hole or two? Are the overflow boxes square or those nice rounded ones?
Rounded or square, my idea would work for both. Albeit the rounded ones being a little trickier. If they have two holes you are set for a dual purpose overflow/water feature/refugium.
One hole would be used with a short standpipe as a drain to a sump. This standpipe should be shorter than your false bottom. Next would be a false bottom made to fit inside the overflow chamber. The other hole would be given a much taller standpipe that would be your water return, and would have to pass through the false bottom. This could be done in both overflow chambers, creating two refugiums/water features/overflows. Plumb your water features any way you like from the top of the overflow chamber, much like a reef tank. Waterfall, trickle wall, stream, a combination, anything you can imagine at this point. Fill each chamber with leaf litter, charcoal, whatever media you like. Drilling boatloads of small holes all through the overflow chamber boxes would allow microfauna to venture out and become food.
Hopefully that makes as much sense to you, as it does in my head.
A 700 gph pump might be overkill even with a couple feet of head. You can dial them back with a ball valve. My concern would be dialing it back to far and possibly overheating/overworking the pump. Using one pump T'd or Y'd to run both features and then dialed back might be the answer.
Cheers,
Scott
Cheers,
Scott.