It's actually not very hard to do and only requires 2 event-lines. You trigger an event which checks if you liked the page (login with fb first) and then returns a code which can be anything you wish. Then you use this code to unlock the content.
It has a disadvantage tho:
Just triggering the event, which checks if you liked the page or not won't unlock it. You need to use the returned code to make it work. (As long as they don't update the facebook object, this will never be possible)
I will write the tutorial this week when I have some more free time. Thanks for your interest.