Hi,

It is possible to customize the barcode app.

Look at the stock_barcode module in the odoo addons. That's where the js/xml files are located for the views. it's not as straightforward as modifying normal backend views, but it can be done.

Most of the data handling logic is located in lazy_barcode_cache and in the js models. 

Here is an example of a barcode app customization:

That was done almost purely in javascript, the only changes to the models required were the addition of 2-3 methods to allow the barcode_cache to be able to fetch all the fields needed.