You can install your XAP as an Out-Of-Browser application using a script to invoke sllauncher
like this:
"C:\Program Files (x86)\Microsoft Silverlight\sllauncher.exe"
/install:"C:\temp\MyApp.xap"
/origin:http://www.mysite.com/ClientBin/MyApp.xap
/shortcut:desktop+startmenu /overwrite
The XAP needs to be available locally for this to work. (I haven't tried it from a network share.)