Ghost circuit should not result in any errors as far as I know and you can run it up to 3 hours at a time. One problem is that if you have an option turned on to minimize run-off (meaning that zone will partially water, pause and water again), you will get "dead" periods within your runtime.
If you need your master valve active for a prolonged periods without interruptions, you could use a 3-way light switch (link), with the outdoor enclosure, or a remote system (link).
If using a 3-way switch, connect one wire from M terminal to one of the brass (shiny mental) screws and SC terminal to the other brass screw. Master valve would than connect to the black screw and you could optionally connect earth ground to the green screw (though this should not be needed, since we are dealing with relatively low voltages).
Switch would than toggle between having the master valve ON irregardless of what Rachio is doing (by connecting SC and Mater valve), and having Rachio control the valve (by connecting M and Master valve).
With the remote, you will need to connect SC and SP terminals to AC/DC terminals of the remote (which one is which does not matter). Master valve wire would than connect to one of the COM lines, M terminal to NC terminal next to the COM terminal you've used and SC should be connected to NO terminal next to the COM. (you should be using either 1st port, or 2nd, not both).
Let me know if you have any questions, I can make a wiring diagram if you need it, but don't want to spend the time if the phantom zone will be your preferred solution.