Kevin,
First off, you must ensure that Active Directory System Group Discovery is enabled on your primary site servers.
Next, you can create a query around the SMS_R_System.SystemOUName that looks for an OU path like: Domain.com/USA/Site/Clients. Keep in mind that it does not use the distinguished name syntax, like OU=Clients,OU=Site,OU=USA,DC=domain,DC=com.
You could also, in theory, use a Desired Configuration Management (DCM) rule to build such a collection. Check the root\RSOP\machine WMI namespace for a class called RSOP_Session, which has a property called SOM. This property contains the distinguished name of the OU that the client is a member of.
Cheers,
Trevor Sullivan
http://trevorsullivan.net
http://twitter.com/pcgeek86