First off, putting user accounts in OU's does not make them more restrictive, it's exactly what the system was intended for. In your case, they would have all the same policies as other users, except with the addition of the screen saver. As for the LDAP thing, I have no idea why Unix would need to mirror OU's, I would think that it's being used as an authentication tool to access Unix resources, so OU placement would make no difference. Anyway, I'm not in the lecturing business, so here's the registry changes you'll need to make:
[HKEY_CURRENT_USER\Control Panel\Desktop]
"ScreenSaveTimeOut"="360"
"ScreenSaveActive"="1"
"ScreenSaverIsSecure"="1"
"SCRNSAVE.EXE"="C:\\WINNT\\System32\\scrnsave.scr"
(this first one activates and protects the screensaver. make sure the path of "scrnsave.scr" is correct for you. also, you can set the timeout to whatever you want, its in seconds.)
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\System]
"NoDispScrSavPage"=dword:00000001
(this hack removes the screen saver tab from the users Display settings)