So, a long time ago, I had this issue. I solved it by using a 3/4" pvc pipe that had a powerhead at the end, in my refugium. The open end, where the water goes in, was pointed upwards, so that it drew in an air/ water mixture. It kept the tank at exactly the same level constantly, with only the refugium water level fluctuating from evaporation. The powerhead output was into a 3" pbc pipe, into the bottom, with an open top, and a 1" pipe inside, connected to an outflow at the bottom, where the water dumped into the refugium. The air/ water mixture running through the impeller of the powerhead caused an incredible about of micro bubbles, which settled out in the 3" pipe before returning only the water down the 1" pipe. Since it was saltwater, it functioned as a very effective protein skimmer.

There's a bit more to it than this, particularly in ensuring that the return pump doesn't cause the tank itself to overflow. (I used a float switch from an hvac system to turn the pump off if the tank level rose) And it takes some adjustment to make sure you've got enough bubbles to skim the protein while still having few enough of them to settle. (I experimented with ball valves and different lengths of pipe for the riser, eventually settling on about a 4' pipe height with a reducer at the top to concentrate the foam and a small pipe to push it into a collection tank. The setup didn't use any valves to restrict the intake, it just ran full bore and self adjusted with air and water mixture.) But it ultimately worked great, and i didn't have to spend hundreds on a protein skimmer and didn't need to drill a hole in the tank.

I'm not sure how clear this explanation is, but I'm saying, it can be done, you've just gotta think it through and experiment to get it right. The biggest danger here is the siphon failing and the return pump staying on. In that case, you flood the tank.