Doing it that way would restrict the overall flow and negate the larger supply pipes. Put the put in the return line where both the inlet and outlet are the 1/2". Often, they put a T on the drain of the WH and run the return line in there when you have a dedicated line for it. So, you'd take the 1/2" tap off the furthest point, run it back to the WH and run it into the bottom of the WH. The system would come with a check valve to prevent water going the other way.