A set of UBL 2.1 schemas with individually extensible ABIEs is found here:

https://www.oasis-open.org/committees/document.php?document_id=62585

In the "val/" directory you will find order-test-good.xml that has a custom extension at the line item level. Running "test.bat" or "sh test.sh" validates the document.

The "xsd/" and "xsdrt/" are as expected, except that the Aggregates fragment now allows <ext:UBLExtensions> on every ABIE.