I came up with an alternative solution. Because I had only one zone for which I didn’t want my booster pump to run, I added one normally-closed relay to that zone that interrupts the pump circuit when that one zone is running.
The relay I used was a Jard 92380. The core is wired to the target zone and common, terminal 5 goes to the M terminal on Rachio and terminal 6 goes to the wire to the pump relay. This works perfectly, but I still think this feature should be on the roadmap.