A guy that works at a LFS near me did this with his 14G Biocube and told me how but it was weeks ago and I haven't brought it up to him since... maybe I should stop by and ask some questions...
He said it wasn't all that bad to do. It's just once you do it you can't go back.
dude....built in water fall..... I am dying for one of those but to damn expensive!
I would set it up the same way as any other viv. Might have to seal in the vent at the top though but from there.... false water bottom build up some scenery to form the water fall. organic top soil, plant. enjoy.
you'd probably need to take out the whole filtration system
It definitely would not work for a waterfall because it skims water near the surface.
Thats probably why he said you can't go back after modding it
Just take out the back wall with the baffles and you have a nice looking curved front tank
Also, you might have to put mesh over the air vents to keep fruit flies from escaping
I feel like it would be easier for you to just get a bowfront aquarium of the same size and make a custom canopy for it. It's seems like a lot of hassle to mod the Biocube plus it's a waste of a system that was designed for a very different purpose. Now this is all if you're going to purchase one at full price to mod it; if you're getting a really good deal or already have one, then I don't see it as being quite as much of a waste.