I got this working – I have installed View Future Posts, and followed the instructions in this comment, but the instructions below made all months containing future posts show up in the sidebar.
Open wp-includes/template-functions-general.php, and search for:
$arcresults = $wpdb->get_results("SELECT DISTINCT YEAR(post_date) AS
year, MONTH(post_date) AS
month, count(ID) as posts FROM $wpdb->posts WHERE post_date < '$now' AND post_status = 'publish' GROUP BY YEAR(post_date), MONTH(post_date) ORDER BY post_date DESC" . $limit);
change it to:
$arcresults = $wpdb->get_results("SELECT DISTINCT YEAR(post_date) AS
year, MONTH(post_date) AS
month, count(ID) as posts FROM $wpdb->posts WHERE post_date <> '$now' AND post_status = 'publish' GROUP BY YEAR(post_date), MONTH(post_date) ORDER BY post_date DESC" . $limit);