Your original question brings up some interesting possibilities.
the monthly adjust plugin was originally written as a quick example plugin when the plugin system was first introduced. It adjusts a variable that controls the amount of time each zone runs on a global basis.
I think it would be fairly easy to modify the plugin to control a single zone. If your turf’s schedule is in it’s own program, the modified plugin could adjust the irrigation time for that program on a monthly basis.
Let me think about it a bit more.
EDIT:
We have been testing a new feature that can cause a plugin to run a function when an event occurs in the main program including when zones are turned on or off. A plugin could check if a particular zone has been turned on and adjust the irrigation time for that zone (I think).
Dan