I've seen that done, not that that helps much-- at <a href="http://www.drivethrurpg.com/catalog/index.php">DriveThruRPG</a>, specifically, which I'm reasonably sure is running an osCommerce version or derivative. (Wish I could be more help than that.)
On my own problem... I finally discovered that the ultimate problem was that the last_modified wasn't setting at all, presumably some sort of problem in checkout_process.php. The section modified by the download controller,
'last_modified' => 'now()',
'date_purchased' => 'now()',
should be setting last_modified identical to date_purchased. Instead, date_purchased sets, but not the other. I'm utterly and completely baffled by this. Theoretically this query still ought to work - at least everything I read of it in tep_db_perform seems in order.
I finally solved it by the inelegant workaround of adding
tep_db_query("update ". TABLE_ORDERS . " set last_modified = now() where orders_id = '" . $insert_id . "'");
just after the $insert_id below the array, and that worked. If anybody has <i>any</i> clue why the original variant might not work, I'd love to hear it - things that inexplicably don't work are usually a bad sign.