Longest Idle would not be possible; however, Circular would be if you use an external XML or SQL database to track the DN the previous call was routed to. Each time the script is executed you would need to check the data source for the last-routed DN and likely use a Switch step to determine what the action should be based on the result. You would also need to update the data source for the next time the script is executed. One note here is that if two or more calls happened to occur at the same time (and query the data source simultaniously) you would likely see both calls route to the same DN. This isn't bullet proof.
As a side note: I'm not aware of any Cisco IP Phone model requirements for hunt group membership. Directory numbers are added to line groups, not phones. Have you actually tried this and not been able to make it work?
