You could use the Asterisk integration with calendars , actually it makes sense to do it in the backend, not in a frontend like FOP2:
https://wiki.asterisk.org/wiki/display/AST/Calendaring+Dialplan+Examples
With that, it would be possible to write calendar entries to fire/dial a custom dialplan that can fire an userevent or similar to notify FOP2 about the change, perhaps via a plugin.
