In the most recent updates of Zello for Android (v. 1.23) and BlackBerry (v 1.21) clients we added experimental support for offline messages. Now when someone is off the coverage or doesn't have the app currently running, you still can send them a message (if they run the latest Zello).
The contacts capable of receiving offline messages will show as 'Standby' in your list with a green circle status icon. The messages sent to such contacts are delivered as soon as the contact returns online.
Let us know what you think and don't forget to tell you friends to update their Zello!
P.S. We are working to enable this feature in Windows and bring it to Zello Cloud. Upcoming Zello for iPhone will support offline messages too including push notifications about new messages.