How about this:
1) Set up an account code with a restricted COR/COS. - export it to CSV named "Restricted.csv".
2) Change the COR/COS to be unrestricted. - export it to CSV named "unRestricted.CSV".
3) Set up a schedule to upload the "Restricted.csv" file at start of times when it should be restricted.
4) Set up a schedule to upload the "unRestricted.csv" file at the start time when it should not be restricted.
You'll still have to use an account code but the schedule will change the COR/COS based on time so it can only be used during certain times of day.
Ralph
This sparked something in my mind. If it's just a single devices it could be on a Business Schedule that kicks it to a restricted COR based on time of day. I guess that's not really explicitly allowing staff to lock/unlock though. You could have the staff phones be able to put the system in day/night mode which would allow use of the device on demand - if nothing else depends on that feature.