That is a good idea, my only challenge with that is the cheep plastic valves that one finds at the bottom of most water heaters... open them up and getting them closed could be a question..
(copied from another forum) The cheap plastic cap that comes with a daisy chainable lawn sprinkler (you should also be able to buy the cap separately) works fine on the water heater drain valve provided you did not lose the washer that came with the cap.
You do have to be able to get the valve itself almost closed first.
Do you get coverage you can count on? From the bag of fertilizer? From your lawn sprinklers?