Just leave the downflow chamber empty. In all respect, bioballs submerged are, yes, far, far better than wet/dry, but imho they don't have the outer-layer inhabitability that lr has, and they still wouldn't be my own choice.
I've used bioballs, microsand, floss, layered media, sponge, you name it, and none of those, again imho, are worth the hassle of cleaning them and having the nitrates bounce around. None of the above is good for a reef: some do use mechanical filtration in FOWLR tanks, but because of the interference these elements provoke in the nitrate cycle, I still wouldn't. Just up your flow and have plenty of live rock, would be my vote.