Get-WmiObject -namespace root\ccm -class sms_client
Here's a screenshot of the output I got from an up-to-date computer (note the version number):
Using that I was able to construct the following query for my WMI filter:
select * from SMS_Client where ClientVersion < "5.00.7804.1000"
I applied the filter to our SCCM client install policy and now it only runs on machines running client versions older than the one specified. Nice!
There are several ways to do the GPO-based install, I chose to go with running ccmsetup.exe as a startup script in Group Policy Management at the following location:
"Computer Configuration -> Policies -> Windows Settings -> Scripts (Startup/Shutdown)"
I set it up to run the following install string:
ccmsetup.exe /MP:<MySCCMServerName> SMSSITECODE=AUTO