If you go to the Client list, you will find a drop-down at the top that defaults to 'all'. You can change this to filter on 'all clients with a policy'. As far as I can tell, this filter ignores the time-based filter (last 2-hours, last 30 days, etc.). When I switched to the 'last 2 hours' view, I can still see clients with a policy assigned that haven't connected to the network in the last few days.
I don't have any clients with a policy that haven't been online in the last 30 days, so I can't test your exact scenario.
If that doesn't work, I would try using the 'Add Client' button (on the same page). You normally use this feature to assign a policy to a device that has never connected to the network, but it might support your need as well.