It will be tricky to design an add-on circuit to use OpenSprinkler with DC latching solenoids. I would recommend keeping an eye on the upcoming OpenSprinkler Bee:
http://rayshobby.net/cart/osbee-10
which is designed to operate on battery and interface with DC latching solenoids. I’ve just ordered semi-final PCBs and expect to roll it out within a month or two (finally!)