The firmware already does this. Handling of rain sensor is in a function called process_dynamic_event, it applies to all programs regardless of whether it’s a program that just started or a program that has started for some time:
https://github.com/OpenSprinkler/OpenSprinkler-Firmware/blob/master/main.cpp#L1200
However, it will be ignored if the program is a manually started program (i.e. not a program that automatically started by its start time), and it will be ignored for zones that are set to ‘Ignore Sensor’.