Possible solution:
Make an override of AdminOrdersController.php located in controllers/admin. Find the line where this starts: "date_add"
add the following below that section:
'date_upd' => array(
'title' => $this->l('Most recent status'),
'align' => 'text-right',
'type' => 'datetime',
'filter_key' => 'a!date_upd'
),
This seems to be working but I'm not sure if this is entirely the correct way to do it.