I didn't find any configuration option that allows for something like that. It might be easier to have two different SSID's on mutually exclusive schedules, such that the no-password one appears during certain hours, and during the other hours the password-based one appears.
The only troublesome part is you have to teach your users two SSIDs, but once their devices learn them, they should automatically join the right one.
Without a ZoneDirector, there's no formal notion of a guest network or scheduling. Of course, if you use something behind the Ruckus (e.g. some sort of firewall / UTM / gateway) to provide the captive portal functionality, it could also potentially have this feature.