How about distributing the app for free on the app store.

When the app is opened, prompt the user to enter a secret code that you've distributed only to Kickstarter backers.

And for non-backers, set up an in-app purchase as an alternative method of unlocking the app.

It's possible though that an app that isn't functional without an unlock code or without making an in-app purchase might violate Apples' rules in some way.