with a drain hole, what would make water move through it? If there's a pressure differential, water would flow to equalize, but once equal it will cease to flow. If you can get new water in the top an equal amount would try to flow out the bottom. Just adding air would keep DO up, but you need water out to carry waste. The small tank guys can advise from experience, but just because you set a tub in a pond, that wouldn't make water flow in or out a single hole.
Hrm. Perhaps a small ejector design.. use the air to create a small water pump. Could you slightly tip the tub, inject a water/air mixture in the drain hole and let excess water spill over the top? Netting to keep them in. Wouldn't get sunk solids out. Would have to go out the drain to get any solids out at all, but getting water up over the lip would be harder than simply pushing it from outside the tub to inside thru drain hole.