Whenever I've needed to install a return line through the drain port of a heater, I've always done it with a tee -- one side goes to the heater, one side comes from the recirc line, and the other goes to a drain. I don't know if it's allowed your way or not, but were to come across a setup like that I'd be very worried about swapping the heater out and not making a big mess without being able to drain the water through a garden hose out of the house.