Hi Ben,
The UBL package is licensed with a closed-source license, because it would be interesting for customers as part of our "enterprise open-source" strategy.
However, there was really nobody interested, apart from one customer...
incompatibility
The package is distributed and installed separately, so it doesn't need to be GPL V2. That's how for example the Oracle DB can work on top of Linux.
However, as I said, there wasn't much interest in the package. Just drop us a note at mailto:info@project-open.com and I can provide you with the source code. There may be dependencies on the rest of ]project-open[, but I believe the structure is quite straight-forward, so that you could adapt it easily. Also, it's only a limited number of UBL messages and only one direction.
Cheers,
Frank