Well, after some more intense research I finally found a viable solution that is clean and simple and works flawless so far.
Just put this into your functions.php to create a custom walker which turns classic permalinks into hashes (e.g. page.com/mypage to page.com/#mypage):
/* Custom nav walker */
class Single_Page_Walker extends Walker_Nav_Menu{
function start_el(&$output, $item, $depth = 0, $args = array(), $id = 0) {
global $wp_query;
$indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';
$class_names = $value="";
$classes = empty( $item->classes ) ? array() : (array) $item->classes;
$class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item ) );
$class_names=" class="". esc_attr( $class_names ) . '"';
$output .= $indent . '<li id="menu-item-'. $item->ID . '"' . $value . $class_names .'>';
$attributes = ! empty( $item->attr_title ) ? ' title="' . esc_attr( $item->attr_title ) .'"' : '';
$attributes .= ! empty( $item->target ) ? ' target="' . esc_attr( $item->target ) .'"' : '';
$attributes .= ! empty( $item->xfn ) ? ' rel="' . esc_attr( $item->xfn ) .'"' : '';
if($item->object == 'page')
{
$varpost = get_post($item->object_id);
if(is_home()){
$attributes .= ' href="#' . $varpost->post_name . '"';
}else{
$attributes .= ' href="'.home_url().'/#' . $varpost->post_name . '"';
}
}
else
$attributes .= ! empty( $item->url ) ? ' href="' . esc_attr( $item->url ) .'"' : '';
$item_output = $args->before;
$item_output .= '<a'. $attributes .'>';
$item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID );
$item_output .= $args->link_after;
$item_output .= '</a>';
$item_output .= $args->after;
$output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args, $id );
}
}
Then, where you want your menu to appear, put the following code to call your menu (which you should have defined as a menu position before in your functions.php), and adjust the options to your liking, apart from the ‘walker’ part:
<?php
wp_nav_menu(array(
'theme_location' => 'onepage',
'echo' => true,
'container' => false,
'walker'=> new Single_Page_Walker,
'depth' => 1) );
?>
— that’s it already. Now you can fill up your menu in the backend without caring about hashes anymore :).