I was tinkering around with doom3 stuff and got something like this working
source code:
https://d3test.svn.codeplex.com/svn/Bot/
usage: compile and zip the dll as well as binary.conf file (can be copied from game00.pk4) as "zeta.pk4" (the filename can be changed in Export.cpp however it must be something alphabetically latter than the original pk4 files), put the created zeta.pk4 file into base directory and launch the game.
EDIT: updated the URL; M$ codeplex's SVN is buggy (which is not really SVN but M$'s Team Foundation Server) and causes problems with props/branches/merging/large commits/etc.