I suggest that you simply remove the on-board PTC fuse to cut off the path from the DC-DC converter to RPi. This way, RPi is completely powered by microUSB cable, while 24VAC will still power the sprinkler valves.
Ray, or someone, can you point me to that fuse? Right now, I’m powering the pi off of OSpi, and powering the screen with USB, but I think I’ll power both the screen and pi off usb, and OSpi with 24VAC.