Option 1. Have you considered using a trigger from IFTTT to send something to ST? I took a quick look on the IFTTT Rachio commands and they have some for flex schedules at the very bottom. In a simplistic form you could have IFTTT “turn on” a virtual switch which CoRE could look for every morning say at 3am. If it see’s the switch on CoRE could open the valve at your preset time for you and follow up by closing it later that day. I don’t use flex schedules with Rachio so I’m not sure if the IFTTT even trigger would work but it’s a thought to explore.
Option 2. Not sure how many zones you use and have available on your Rachio but I’ll use mine as an example. I have the 8 zone unit with 4 zones used. You could set up the an unused zone to mimic a real one; in my case one for one. Setup Rachio to run flex schedules on those “virtual” zones say at 1 am. When ST see’s this virtual zone fire that could be used to open the main valve for you. Setup a timer so it automatically shuts it off later that day too for you. Instead to be safe with a leaking pipe I’d probably setup a basic daily schedule in ST to try and turn off the valve everyday at say 10am if you water in the pre-dawn hours. This way you never have to worry about did I shut off the water valve kinda thing. With the valve turned on from the virtual zone(s) the real zone(s) could run like normal from Rachio.
Option 3. Depending on how bad your leak is and assuming you have a fixed watering start time each morning you could simply have ST turn the valve on every morning 15 min before a watering would start. Then if Rachio starts a cycle your good to go and ST could turn the main off on the set schedule. To compensate for unnecessary leaking when no flex is running I’d setup a CoRE or otherwise that says at say 5:10 am (assuming your cycle starts at 5am) that looks at if no Rachio zones are active and the main is on then to turn off the main.
Hope this helps and works for you or at least gets some alternative approaches thrown out to maybe spark an idea. Good luck.