If this could be integrated to read files like the ones used to by this addon and override existing ones.
That would be fabulous, since apparently according to sources on that site. Valve has patched/blocked out mission.txt/script files/population txt files from working on official campaigns.
However, the recent Valve update showed a way to get custom population.txt files working again.
Combine that with this great extension, and 2 out of 3 are working again.
2. Unlockable melee weapons on all campaigns (though it is sadly missing the female boomer on L4D1 campaigns)
The 3rd item missing from that list is being able to use modified talkers again, which is what that addon link is.