You are tapping into the cooling system on the hot side with the heater connection. That hot water won't be effective cooling and may actually warm the fluid up.

I don't know how or where you would find a pressurized source of cool water pumped from the radiator bottom tank... you could somehow tap into the cool water feed from the pump into the block. Wouldn't be pretty and may throw the engine cooling system out of balance.

I think I'd wait for the other cooler (hopefully a good size unit to go in front of the radiator).