An interesting project.
You will need to have the OSPi on a battery backup so it will not be effected by power outages. It does not draw a lot of power so that should not be a big problem. Perhaps a small solar panel would help.
I think Scott’s suggestion of running the zones in ten minute intervals is a good idea.
The latest versions of the OSPi boards have analog inputs and a small relay included. The analog inputs could probably be used to monitor power to the pump and the relay could be used to start the pump.
You could write a plugin for the Python program that would monitor elapsed time for each zone and re-schedule irrigation after a power outage based on programmed duration minus elapsed time. The software is designed to make custom features easy to add.
Ray will probably have some interesting suggestions.
Dan