How to alter menu parent select list on node form?
Menu module does form alter to add the Menu Fields to the node creation form using function menu_form_node_form_alter in menu.module

Check your custom module weights to catch this data in your custom module to do form alter...

There is a new drupal 7 API called hook_module_implements_alter() which lets you alter the execution order for a given hook WIHOUT altering the module weights table.