I know what you mean. The hole will create an oxygen bubble when the water level gets too low and it should stop the suction of the refugium from pulling the water back into the dt.
BUT
unfortunatly that idea wont work for me and would be useless because of the fact that the highest point of the return pipe is still way to low below water level in the refugium that by the time the water level gets to the hole about 6" will have to flow into my dt which would mean it will still over flow. It usually stops at that point or before it anyways, so there is no real point in drilling.
I think the HOB refugium that I bought was just poorly made. The part where I hang it on should have been a lot lower so that the water level of the refugium met the water level of the tank, so there would be no gravety forcing the water into the dt if the pump fails.
The next suggestion will be, "why dont you make the water levels the same then" , I cant do that either because then the water wouldnt be able to clear the bubble wall and the refugium wouldnt work. Also I'd be loosing half the volume of water in there because of it.
I think my emergency plan is the only way to fight the dt overflowing. %%
Thanks for the help though! If you think of a different way, let me know! I gues its harder for you to think of it sin ce you cant see the whole set up properly, pics just dont do justice!