It's an experimental area. I will experiment with a rufug, reverse light cycles, O2 levels and nitrate/phos. levels. It may or may not last and just end up as an empty space in the sump.
Empty space is still evaporation reserve. Or a decent place to toss and hold cuttings while you sell them etc or fish, soak driftwood etc, add a UV for the green water, or another mechanical filter etc.
If done right, even for a reef, you'll end up adding NO3/PO4 or more fish food etc.
Reverse cycles really do not work, as the CO2 is required for O2 addition and the gain is relatively small(100% vs 125-130% if you are a good grower). Emergent growth= no added O2, so you are stuck with adding CO2 no matter what and only for a small area vs the main tank. So you need to add lots of CO2 to both the refuge and the main tank for very little gain. That's not worth it no matter how it's sliced.
Reefs? A little bit for pH management but there's no CO2 added to the water, maybe just the Kalk/Ca reactor but then it's degassed prior to adding back to the main tank.
If the tank was 100% non CO2, well, you could scheme a system that would do okay, but you'd need some space between the overlap for the best results.
I'd suggest a tray system with pots, much like those window herb gardens and see how plants fair with the crowns in the water in various dirts/sediments and then the rest above water.