“Can I wire up my 12VDC power (solar/battery) directly to the DC version of OpenSprinkler..”
– Yes, you can. 12VDC is slightly on the high side (which means the valves will be drawing more current than necessary), but most valves are very tolerant to a wide range of operating current, so it should be fine. There is no need to step 12VDC down to 9VDC.