Welcome Ed. There are a couple of ways to handle this. You can use a UDD (Unmanaged Device Discovery) for a subnet to detect machine without the client. The issue here is to have them automagically. There was a script to do this on DroppedPackets.org but the link is broken.
You can deploy via GPO http://community.landesk.com/support/docs/DOC-1199. This way you can use GPO WMI filtering just to a certain subnet. Not an expert on Directory Service but seen WMI filtering done.
Can use a login script with logic for the subnets
If you currently have another desktop management system you can use it to push out the client.