Do you want to run Software Update Services through a proxy server? In the /System/Library/LaunchDaemons/com.apple.swupdate.sync.plist file you can add the following (if your proxy were 10.0.0.5:8000:
<key>EnvironmentVariables</key>
<dict>
<key>http_proxy</key>
<string>http://10.0.0.5:8000</string>
</dict>
