Still reading through the documents. Some of this stuff is quite trippy - an underground war between dwarfs with scrap-firing gravity guns and blind dogs with a talking leader... The Zone was originally envisioned as a much more dangerous place than any player has experienced.
And it seems I shall have to eat my hat, because it looks like Final Day was at some point intended to be an in-game faction. If I'm reading this right, they were based in the marshes and studied zombies and psi-emissions, believing that the Zone would soon destroy the world through its uncontrolled expansion... Sort of a nastier counterpart to Clear Sky, I guess.
I've got build 1580 working. Be advised that it seems to be missing the ui_icon_equipment.dds texture and will CTD if you open the inventory screen. Substituting the icons file from build 1935 seems to work.