How to get the value of a disabled form field in webform civicrm?
Unfortunately it's a limitation of the Webform module (not specific to Civi) that hidden fields cannot be accessed as tokens or from hooks. They might be available in the $_POST superglobal, but only immediately after submitting page 1 (e.g. not after returning to page 2 from page 3, or from a saved draft, or from reloading page 2 with validation errors). Since you are writing php code, consider using the CiviCRM API to fetch this info instead.