Default text in a specific post type, but not visible?

You can manually hook into the save_post action and update the post’s content on save. Here’s a simple piece of code that does it:

add_action( 'save_post', 'update_post_content' );
function update_post_content($post_id) {

    // If this is a revision, get real post ID
    if ( $parent_id = wp_is_post_revision( $post_id ) ) {
        $post_id = $parent_id;
    }

    // Get the current post type
    $post_type = get_post_type($post_id);

    // Check if it's a custom post type
    if ( 'posttypehere' != $post_type ) return;

    // unhook this function so it doesn't loop infinitely
    remove_action( 'save_post', 'update_post_content' );

    // update the post, which calls save_post again
    wp_update_post( 
        array( 
            'ID'            => $post_id, 
            'post_content'  => 'content here' 
        ) 
    );

    // re-hook this function
    add_action( 'save_post', 'update_post_content' );
}