How does it relates to synchronization? By the way, you can already password protect your synced data.
To avoid people having access to Opera and any other software or data in your computer, just use a password to login into your OS and add users for others to use it when needed.
You can also use "Custom New Tab Page" add-on to override New tab page with any URL, including your favorite add-on new tab page URL. I'm using it with my Group Speed Dial add-on and it works well.