We use something very much like this in our environment. I've taken a code snippet from our function file and login scripts. Inside the InstallSUS function are comments to indicate time of day, what to do etc. I altered slightly
from our environment so make sure you test before deployment!

All the registry information comes from SUS deployment white paper at Microsoft. Adapt for your needs.
Code:

;This part would go in the regular section of your login
;script.
Select
 Case InGroup("FirstGroup") ; First AD group pointing to SUS 1
  InstallSUS("YourFirstSUSServer") ; SUS server 1 no HTTP://
 Case InGroup("SecondGroup") ; Second AD group pointing to SUS 2
  InstallSUS("YourSecondSUSServer") ; SUS server 2, no HTTP://
 Case 1
  ?"No SUS configuration for this group."
EndSelect

;Here to the end you can put at the end of your script
; or you can put it into a separate file and use 'call'
; command to load it before script processing. This
; part actually does all the work.
Function InstallSUS($LOCALSUS)
 $SUSCONFIG=ReadValue("HKEY_LOCAL_MACHINE\Software\Policies\Microsoft\Windows\WindowsUpdate",
  "WUServer"
  ) 

 If $SUSCONFIG="http://$LOCALSUS/"
  ?"SUS is already configured at server $LOCALSUS."
 Else
  ?"Setting up SUS for your computer."
  ?"Your SUS server is $LOCALSUS."
  ?"Stopping Windows Update Service"
  Shell "%COMSPEC% /e:1024 /c net stop wuauserv"
  ?"Writing parameter values."
  AddKey("hkey_local_machine\software\policies\microsoft\windows\windowsupdate")
  AddKey("hkey_local_machine\software\policies\microsoft\windows\windowsupdate\au")
  $NUL=WriteValue("hkey_local_machine\software\policies\microsoft\windows\windowsupdate\au",
   "RescheduleWaitTime",
   "00000005",
   "REG_DWORD"
   )  

  $NUL=WriteValue("hkey_local_machine\software\policies\microsoft\windows\windowsupdate\au",
   "NoAutoRebootWithLoggedOnUsers",
   "1",
   "REG_DWORD"
   )

  $NUL=WriteValue("hkey_local_machine\software\policies\microsoft\windows\windowsupdate\au",
   "NoAutoUpdate",
   "00000000",
   "REG_DWORD"
   )

; Automatically download AND install updates
  $NUL=WriteValue("hkey_local_machine\software\policies\microsoft\windows\windowsupdate\au",
   "AUOptions",
   "00000004",
   "REG_DWORD"
   )
; Deploy every day
  $NUL=WriteValue("hkey_local_machine\software\policies\microsoft\windows\windowsupdate\au",
   "ScheduledInstallDay",
   "00000000",
   "REG_DWORD"
   )
; Install time 13:00
  $NUL=WriteValue("hkey_local_machine\software\policies\microsoft\windows\windowsupdate\au",
   "ScheduledInstallTime",
   "0000000d",
   "REG_DWORD"
   )
  $NUL=WriteValue("hkey_local_machine\software\policies\microsoft\windows\windowsupdate\au",
   "UseWUServer",
   "00000001",
   "REG_DWORD"
   )
  $NUL=WriteValue("HKEY_LOCAL_MACHINE\Software\Policies\Microsoft\Windows\WindowsUpdate",
   "WUServer",
   "http://$LOCALSUS/",
   "REG_SZ"
   )
  $NUL=WriteValue("HKEY_LOCAL_MACHINE\Software\Policies\Microsoft\Windows\WindowsUpdate",
   "WUStatusServer",
   "http://$LOCALSUS/",
   "REG_SZ"
   )
   $WUAUSERV=ReadValue("hkey_local_machine\system\CurrentControlSet\Services\wuauserv",
    "Start"
    )
   If $WUAUSERV="2"
    ?"Automatic Updates already set to start Automatically."
   Else
    $NUL=WriteValue("hkey_local_machine\system\CurrentControlSet\Services\wuauserv",
     "Start",
     "2",
     "REG_DWORD"
     )
   EndIf
   ?"Restarting Windows Update Service."
   Shell "%COMSPEC% /e:1024 /c net start wuauserv"
 EndIf
EndFunction