"Read only" iPhoto library archive
Unfortunately iPhoto refuses to open library files you have no write-access to. As soon as you put the file in a write-protected image/device or lock the file, iPhoto will display an error that it can't open the library.

Therefor it seems your only option is to keep one writeable, and if you're paranoid of accidental changes put a none-writeable copy somewhere else (be it any physical media or a locked file).