Use in-app for trial?
Apps that are “beta”, “demo”, “trial”, or “test” versions will be rejected
  
  Apps that unlock or enable additional features or functionality with mechanisms other than the App Store will be rejected, except in cases where the application hosts plug-ins or extensions
  
  Apps containing “rental” content or services that expire after a limited time will be rejected


Doesn't seem like it's allowed.

Source: App Store Review Guidelines for iOS apps (login required)