Realistically few end users are going to be interested in modifying Rays firmware. But it is easy to control open sprinkler externally. Using the program functionality in OS is probably the safe way, as OS is then responsible for valve closing.
While irrigation wouldn’t normally be run from a pressure tanks, I’m sure some drip systems are configured that way. A drip system could be run many times per day with a long station delay assuring adequate water pressure for other uses.
My previous post said “mechanically setting the rain flag”. I should have said “activating the rain sensor”: As in activating the rain sensor circuit as a system not ready indicator. Perhaps another way would be to programmatically alter the rain flag.