apply custom where to the default $wp_query

The posts_where filter should still fire. Just add it like this:

function homepage_load_posts($query) {
  if ($query->is_home()) {
    add_filter( 'posts_where', 'filter_where' );
    //

And make the filter self-removing…

// Create a new filtering function that will add our where clause to the query
function filter_where( $where="" ) {
    remove_filter( 'posts_where', 'filter_where' );
    // posts  30 to 60 days old
    $where .= " AND post_date >= '" . date('Y-m-d', strtotime('-60 days')) . "'" . " AND post_date <= '" . date('Y-m-d', strtotime('-30 days')) . "'";
    return $where;
}