Can I use Android Zello with the screen turned off?
First of all, set desired hardware button as a PTT button (Options>Push-to-talk button)
Next, activate desired contact in Zello app. Now when screen is turned off, you can still use hardware ptt button
to send a message to selected contact.
Alternatively, you can use a wired headset button for the same purpose. There's no way to switch between contacts while screen is off so you all messages will be sent to the same contact.