This is a simple VQmod that I wrote that will add a "In Stock" or "On Order" message to the drop down for product options. I've tested with with the default theme and BurnEngine on 2.3.0.2 and it works on both themes. No core files are modified and the xml is easy to understand if you want to edit the message shown.

Note: The option needs to have Subtract Stock set to "No" in order to show. If the option has Subtract Stock set to "Yes", the option is not shown.