Maybe I'm missing something here, but wouldn't it be easiest to do this via the scheduler?
First, create one script to disable the property, and then another one to enable it:
/system script add name="DisableRADIUSMAC" source="/interface wireless security-profiles set 0 radius-mac-authentication=no"
/system script add name="EnableRADIUSMAC" source="/interface wireless security-profiles set 0 radius-mac-authentication=yes"
And then add two scheduled jobs running at 9am and 4pm, firing the scripts:
/system scheduler add start-date=aug/14/2009 start-time=09:00:00 interval=1d on-event=DisableRADIUSMAC
/system scheduler add start-date=aug/14/2009 start-time=16:00:00 interval=1d on-event=EnableRADIUSMAC
Hope that helps,
Felix