OK, it seems that a very simple addition to showgroups.php will do the trick...
In showgroups.php, insert the line in BOLD where you find:
Code:
// get moderators **********************************************************
$moderators = $db->query_read("
SELECT $locationfieldselect user.*,moderator.*
FROM " . TABLE_PREFIX . "moderator AS moderator
INNER JOIN " . TABLE_PREFIX . "user AS user USING(userid)
INNER JOIN " . TABLE_PREFIX . "userfield AS userfield USING(userid)
WHERE user.usergroupid = XX
#ORDER BY user.username
");
Replace XX with the usergroup ID of the Moderators group. Works for me on 3.5.2, but I don't think there's any changes between the latest versions of this file.