If you don't want to update core files, you can write a plugin that adds a widget to the client dashboard similar to the Invoices, Transactions, and Services widgets.
Otherwise, you'll need to update core files to make the data you want available to that template.
To do so, open the ClientMain controller in /app/controllers/client_main.php.
Find:
$this->set("client", $this->client);
Above that line add:
$this->set("max_due_currency", (isset($max_due_currency) ? $max_due_currency : null));
$this->set("max_due_amount", (isset($max_due_amount) ? $max_due_amount : null));
Open up the client_main.pdt template you mentioned, and use the CurrencyFormat helper to format the amount due, e.g.:
<?php
if ($this->Html->ifSet($max_due_currency) && $this->Html->ifSet($max_due_amount)) {
?>
<p><?php echo $this->CurrencyFormat->format($this->Html->ifSet($max_due_amount), $this->Html->ifSet($max_due_currency));?></p>
<?php
}
?>