The UBL standard seems to be very well documented: http://ubl.xml.org/ (like Dirk has already pointed out).
There are many different schemas and samples available and free for download and usage.
I have quickly tested to read an invoice file in xml format.
Therefore, I have imported the invoice schema from XSD into a Bridge test project.
All dependencies have been resolved properly and the file was read successfully. All elements are available.
So, we can process UBL from a technical point of view (XML based).
From the functional side, it looks not very different compared to other standards for business documents.
We could gather the mappings from/to the source/target format together in some workshops.
Just let me know if you need any additional information.
We could also test with actual data, if you already have sample files available.
I can also provide you the test project for the invoice processing.