dynamic query based on unassigned value?

I've assigned a custom property "app_tier" (integer) to my application monitors, and now setting up dynamically queried groups for Tier 1, 2, etc.

How do I query an "unassigned" condition, for objects that don't yet have that custom property set? "is not" doesn't work, because of the "or" relationship between conditions, and because it'd become too complex to manage should the property have a lot of values.

Thanks for any ideas!