Aw so you don't get to really enjoy the 40gal huh haha. 10gals are cool but you keep wanting more space. At least i do with mine!
I might mod one like this after my 40 is setup and go towards a more softie setup. SPS will be in the 40.
You sure a SCWD will fit in the back area?
This design is a tried and true method i have seen done on numorous occasions, even have a tank ready to be modded like this myself. I think it should work out great. I'm guessing some MH light will be lighting the fuge as well?
Another option that saves pipe and hose visability is just pushing the water of the return straight through the wall. Like mount the pump/powerhead in the wall and go that route. That's easy assuming you are using acrylic for the false wall so you can drill easy.
Is this going to be a 3 visible side tank or like normal? I'll be tagging along! Hey maybe i'll have some SPS to frag by the time the school year rolls around and you get it all fixed up.