Fast Talkie allows you to send Zello PTT messages from your lock screen using the volume down button. 

Fastest way to start or reply to PTT
1: Press Power button to wake up phone
2: Hold Volume Down to PTT in the previously selected Zello Channel

Setup Required
1: Install Fast Talkie
2: Enable Permissions - Draw Over Apps & Device Administrator
3: Choose a Zello Contact or Channel
4: Press Power button to Lock your phone