It turned out to be significantly easier to find users without an assigned LineURI than I expected. Thanks to Nick Smith for pointing me in the right direction when answering something else. :o)
The Powershell script takes no parameters and will display the primary SIP URI for all OCS enabled users without a phone number (or LineURI). The count displayed at the end is written to the host so it won’t show in the file if you redirect.
$OCSUsers = Get-WmiObject -Query "Select * from MSFT_SIPESUserSetting where LineURI IS NULL"
The output will be correctly formatted for use with OCSAssignTelUri.wsf