Well if you build this under the tank then you might as well turn it into a sump. Probably the easiest way is to put an overflow box on the back of the tank. You will have to cut the back of the hood tho. You could take a chance and drill the back of the tank so you wont have to cut the hood.
As to what you can fit under the tank in the stand.......I am not to sure. There isn't enough room under my 29 to fit a standard ten. That's why I built mine in the middle chamber. If you make a sump or have one built then you could build a pretty tall unit to get more volume. I would not put in anything like sand or rock, just water.
In doing this^^^^you would have to flood the rear chambers and run the water level almost equal to the display. Your evap will be noticed in the sump, so that will be where your topping off water will be added. You will be able to put your heater down there, as well as any other goodies you like to use ie..carbon ect.
Depending on how you plumb this you will need at least one more pump to get the water out of the sump and back to the display. The good news is that for your 14 gallon Bio Cube you will not need to pump any real large amounts of water.
I don't see any benefits of using an Eductor. That is not what they are designed for.
I hope this helps a little bit.
