I have to make a correction from my earlier post. I said:
core library licensed under GPLv2
This is not true. See [1] for the core xdoclet license which doesn't seem to be any standard license. Some other (parts of) xdoclet modules are licensed under GPLv2, however.
The basic question therfore remains: Is there a license compatibility problem in the give case where I package an Apache 2.0-licensed DTD with Java code licensed under GPLv2?
Florian [1] http://xdoclet.sourceforge.net/xdoclet/licenses/xdoclet-license.html