ok, I see what you mean, I may add that in the next version.
In the meantime, you can simply change the 'out of stock' button wording in skin/reboot/customer/main/buy_now.tpl
Code:
<p><a class="btn btn-danger" href="product.php?productid={$product.productid}">{$lng.txt_out_of_stock|upper}</a></p>