I can get a script to automatically enter browse mode and zoom when switching to that layout in preview mode on my Windows 7 system. Perhaps this ia a Mac only issue or perhaps you did not select the check box that enabled the script trigger in preview mode?
I am using Win 7 - and have exactly the same issue - I use an on entry layout script trigger to Enter Browse mode and it does not work.
This means I have to give users access to status bar
Sorry, but have to confirm the obvious: Can you confirm that the check box for Preview Mode is selected for this trigger?