Option 1 is to connect 12V LEDs directly to OpenSprinkler. The reason this is doable is that LEDs are diodes so they will block half of the AC valves. Since OpenSprinkler outputs 24V AC (RMS equivalent to 24V DC), that effectively becomes 12V after half of the AC waves are blocked.
Well, that’s in theory. In practice there are some issues:
1. AC waves have high peak voltages, so I would recommend connecting a power resistor in series to limit the current. Or alternatively, connect two LED strips in series to halve the voltage on each. This will probably make the light dim, but it reduces the chance of damaging the LED due to high peak voltages.
2. You mentioned 12V, 10 – 50 watt range. That means the operating current is a few amps, this is beyond the capability of each zone on OpenSprinkler (800mA per zone), so in practice this will probably make the light a lot dimmer than what it should be.
Option 2 is use the on-board relay to control landscape lighting. This way, the LED can run on its own power source and not depend on the 24V AC sprinkler transformer.