Using Software Update server without binding Mac to Lion Server domain
There is a way to do this, but it involves fiddling with DNS to make a 'transparent' software update server. When clients are on your network, their requests to "Apple's" SWU servers go to yours, without any modification of client settings, it truly is transparent.

Check this article out. It's a little old, but this is the general idea that you want to be going with.


  In your internal DNS set swquery.apple.com, swcdn.apple.com, and swscan.apple.com as CNAMEs for the server running SUS.