reading more about it "Nexuiz is built on the power of the Darkplaces engine, which is a heavily modified version of the original Quake." From the screenshots it looks more like a modified quake2 engine.
But since it is based off a quake 1 engine (which has been open source for some time) there's probably a bunch of people out there that know how to make mods for it

It's also why it doesn't require a high end system

i wish there was info about the weapons, will have to check the forum...