The idea is nice, but I think the main difference between Sui Generis and DayZ is that SG isn't a multiplayer-focused game to begin with.
DayZ relies heavily on the pvp interaction. That doesn't merely mean having a shootout, but also having a truce with someone and temporarily working together. The zombies are indeed no real threat, and were never really intended to be much danger to anyone with decent gear. Not to mention that melee combat in DayZ and ArmA is absolutely rubbish.
Sui Generis is a different story. The multiplayer that is being talked and speculated about is closer to something like Fable or Borderlands, where only a small number of players can be on a server. Sui Generis is also, due to the lack of firearms, a much slower paced game in terms of combat. This would ruin the aspects of betrayal and backstabbing that could possibly occur. Even if you were to create 64-player servers, the game just doesn't fit the format very well. If you turn your back towards someone you've allied with, the potential danger is very limited, due to the lack of a first-person perspective. There is also no way of killing someone before they have a chance to strike back, making the whole dynamic fairly wonky.
What I would suggest, that isn't really the same, is having the following:
On a server with a limit of 16 people, players get spawned in random rooms. These rooms are randomly generated and can be filled with loot and enemies. Seldom will players spawn in the same room, but when they meet up, both players have a clear choice. Will they see their fellow player as an adversary, and wonder what treasures they'll be able to loot from their corpse, or will they see this person as an ally, and settle on a truce, in order to conquer the dangers of this dungeon? This would not only make the pvp interaction a lot more personal, but it would also heavily accent the consequences of killing/joining up with someone. Monsters will be patrolling everywhere, and the choice to kill your ally might backfire as skeletons seem to pour out of the woodwork. This would create a sense of emergency, and last-ditch efforts at saving/protecting those in danger. This would spark stories in the same way DayZ does in its format; experienced players skulking along spawn-points on the map, looking to kill newcomers and take what they have, and other players seeking out these poachers and making the area a bit safer for everyone.
I think that would fit a lot better as a distraction from the single-player "campaign", and wouldn't so much break the dynamic that Sui Generis seems to build in terms of tactical gameplay and the interactions between both players themselves and the world around them.
That's just my two cents, anyways.