I have a Department named Inventory for this. You can set the Department
of a computer while it's unmanaged and set your Smart Group to look for
that Department. Hopefully, that's enough to get you going. Let me know
if I misunderstood you.
Run the following in terminal. This basically resets the Software Update
service and allows new updates to flow through again. sudo launchctl
kickstart -k system/com.apple.softwareupdated Even better, you can make
a Self Service policy that runs that...