I would say go with the text document. It sounds like an absolute requirement, there's no point in risking it. You could do a UML diagram and export it as a JPG image - they should be able to view that without any problems.
in my opinion, you should respect the format expected by Sun for choice.txt (pure ASCII). On the other hand, there is one place where you can add a UML diagram; this is in the javadoc. I have seen sometimes a UML package dependencies diagram at the overview level in well-known java lib javadoc. It gives a quick overview of the packages, and it's really handy for the user.