My thinking is to block off the bulkhead (or close off the drain(s)), let the tank start filling higher and with any luck, it'll swim out when the water level is above the overflow.
In practice it would probably make a whole lot more sense to shut off the return, then close the drains, then manually move water from the sump up to the DT (make sure it's not draining back down through the return pump).
Another idea would be a long thin, flexible piece of plastic that you could push down one side of the overflow and see if it bends to go across the bottom and back up so it's under the fish. You might be able to pull it back up that way.
The idea I think would probably work the best would be to get a bunch of small, heavy objects like marbles or small rocks. Start putting them into the overflow and the fish will have no choice but to move closer to the top. If you can't it once the rocks get close to the water surface, you might have to raise the water level in the DT. Either try what I mentioned at the beginning of the post or even closing off your drain and raising the e-drain stand pipe.
Having said all that...have you tried a net? I would think you could push a net down one side and pin it against the other side. You don't even have to actually catch it, just get it pinned and move the net up towards the top.
And regardless of what method you choose (mine or a different one), I feel like the key here is going to be to use food to coax it where you need it to be.
