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.
Is that the F1 fuse, or, if not where is that fuse on the board?
Any picture available of the location. I successfully soldered in a male header to the OSPi, so I think I can remove the fuse also…