Zero LiPo will assert BCM 4 when the battery starts to run low- you can use this to trigger a soft shutdown, there's no need for OnOff SHIM.
OnOff SHIM is designed to cut the power from an external microUSB supply, and wouldn't bring much to the table in this case.
After soft-shutdown your Pi will fire up again when you reconnect your charged battery.
Our Clean Shutdown script supports Zero LiPo: https://github.com/pimoroni/clean-shutdown
This is set up automatically when you install Zero LiPo via:
curl https://get.pimoroni.com/zerolipo | bash