Here's the setup for this function:
In tpl_product_info_display.php, around line 99, duplicate this line
<?php echo (($flag_show_product_info_quantity == 1) ? '<li>' . $products_quantity . TEXT_PRODUCT_QUANTITY . '</li>' : '') . "\n"; ?>
and modify both similar to the product model line.
<!--bof Product details list -->
...
<?php echo (($flag_show_product_info_model == 1 and $products_model !='') ? '<li>' . TEXT_PRODUCT_MODEL . $products_model . '</li>' : '') . "\n"; ?>
...
<?php echo (($flag_show_product_info_quantity == 1 and $products_quantity >0) ? '<li>' . $products_quantity . TEXT_PRODUCT_QUANTITY . '</li>' : '') . "\n"; ?>
<?php echo (($flag_show_product_info_quantity == 1 and $products_quantity <=0) ? '<li>' . TEXT_MORE_INFORMATION . '</li>' : '') . "\n"; ?>
...
<!--eof Product details list -->
I haven't yet found the variable that holds the out of stock marker text, so I redefined the TEXT_MORE_INFORMATION constant (in /includes/languages/english/my_template/product_info.php) which I don't need, as a temporary measure for testing. You could also add a new define there for TEXT_PRODUCT_O_O_S or something.
I don't plan to show quantity in stock, but may want to note that the item will be made to order if out of stock. For that, I can modify the second line to:
<?php echo (($products_quantity <=0) ? '<li>' . TEXT_MORE_INFORMATION . '</li>' : '') . "\n"; ?>
so the oos text will show if appropriate regardless of the flag setting.