Currently, the UndeleteForm::undelete hook is only passed two parameters (&$archive and Title). However, some extensions, such as Newsletter, may want to log the reason for undeletion, the user who initiated undeletion (not as important. could include, could leave out.), and modify a status object (in order to display errors).
This hook is found in SpecialUndelete.php in the UndeleteForm class, undelete function, starting on line 1771.