How to apply the ‘current_page_item’ class to an archive page in `wp_list_pages()`?

You can use the following filter:

add_filter('wp_list_pages', 'foo_bar');
function foo_bar($html){
    //Create a regex pattern to identify the LI element with preg_match
    //Create another pattern to identify the class attr and alter it with preg_replace
    return $html;
}

As I’m not familiar with the exact elements you are working with, I can’t write up a regex for you. In theory, following these steps should work for you.