Hi,
If you want to keep the stock to zero in these products and have the notification only for the other products, then yes, it would require the customization of the code of the plugin.
An alternative would be to set the stock to unlimited in these products and instead set the sale start date in the future or the sale end date in the past for these products. That way, the add to cart button will still be unavailable for them but the out of stock notification plugin won't take these products into account as their stock will be unlimited.