Dynamix' Mitch Shaw has updated his .plan with word that he's created a new mission type for Starsiege: TRIBES that's currently in QA, and will be released soon. It's a large update, but here's an excerpt that gives the general rules of the mod:

Basic concept: you "freeze" instead of dying. When your health hits zero (or basically close enough to not matter), you get knocked into observer mode around your (still standing) body. Frozen players have a blue armor skin and are stuck in the "using PDA" pose. You randomly throw all the ammo you're carrying and lose your pack when you get frozen. You remain frozen until one of two things happen: your entire team gets frozen (bad) or one of your teammates hits the enemy's switch (good).

If a whole team is frozen, they respawn and the other team gets one point. If a teammate hits the switch, everyone on that team gets unfrozen and the player who touched the switch gets 1 point for everyone who was frozen. You get full ammo for every weapon you're holding when you get unfrozen, and also receive a repair kit. You will take damage for leaving the mission area, just like in Deathmatch. All the missions I've converted for Freeze play were originally CTF maps, and the switches are exactly where the flags used to be (exception: Desert of Frost, the new version of Desert of Death, has the switches inside the pyramids but not inside the holes in the floor.)