Bruno,
The headers (Orbeon-Username, etc.) should be passed whether the persistence implementation is implemented Java or not. If they are not, it is a bug.
This said, we have made some changes as recently as 4.9 regarding the handling of headers:
https://github.com/orbeon/orbeon-forms/issues?utf8=%E2%9C%93&q=milestone%3A4.9+headerSo it might be something that has been fixed already, or who knows, this particular scenario might still not be handled properly.
-Erik