It's not as easy as you'd think mate, I looked into a similar idea as a complete water-cooled-noob and all the advice was that it wouldn't really work. Firstly you're still going to end up needing a big rad as without one your only method of cooling would be constant evaportion from the reservoir which would in turn require frequent top-ups. Also you'll have to filter the crap out of it due to any particles of dirt and dust that will inevitably find their way into the open system. In a design with a waterfall incorporated I'd also imagine (but this is pure speculation) that you'd be left with a dry pump when the system is shut-off, depending where you were able to locate it. Pretty sure that would be bad :p
If there's a way to use one pump to feed 2 different systems though, well that might be a nifty solution! If you could manage to pump 2 individual water sources without significantly increasing the necessary hardware you could run a room-temperature open reservoir and waterfall while also incorporating a standard water-cooling system using the same pump. No idea if there's any kind of pump around that would fit the bill mind so probably a pipe dream
