I was able to build the package with the free Java engine (GIJ) so docbook-defguide can stay in main. However, it was some kind of disappointing: Sun Java needs an hour and a maximum heapsize of 512MB to build the package on my system. GIJ needs 16 hours and a maximum heap size of 1GB to build. kaffe, which I tried too, fails much earlier than GIJ with a maximum heap size of 512MB. So I decided against it and for GIJ.
If you know of a way to speed up the build process (besides the possibility to buy a faster system ... except you want to make it your christmas or birthday present for me *g*), don't hesitate to tell me. The packaging files are in the Debian XML/SGML groups SVN repository.
However, expect the package in your Debian repository soon (guess, my sponsor wants to rebuild it, which may need another 1x hours :)).
Ardo van Rangelrooij kindly offered to build the package on an AMD X2 4600+ with 6GB. The build time decreased to at least 7-8 hours. The package will be uploaded to the Debian archive within the next days.