Well, I would imagine that the ammonium chloride breaks down into constituent ammonium and chloride ions in solution anyway, given that NH4Cl is pretty soluble in water.
Maybe the issue is analogous to turtle anoxia. Certain turtles overwinter in ponds, and since some of these ponds freeze, the turtle is trapped underwater for some time. Given that their metabolic rates slow immensely, and gas exchange occurs through the cloaca, the major issue here is a buildup of Carbon Dioxide and resultant carbonic acid. Turtles use the bones of the shell as a "carbon sink" and therefore can tolderate several months' worth of CO2 buildup without suffocating.
Now, maybe there is an extensive mod of the Archi's excretory system that can serve as an ammonium sink. They use pretty well-developed metanephridia, but... Hmm...
Wait one second!! The major ion concentration gradient is in the mantle, right? Fish use their passive and active diffusion across their gills as a way to excrete excess ammonium and other ions, right? Do the ctenidia of Archis also serve such a function? Such a convergence with fish would not be out of the question. It just seems that continuous ion-pumping would get to be pretty metabolically costly for an animal that lives as an ambush predator. Then again, its not like its hunting gazelles on the Serengeti.
Wow, we really don't know much about these animals, do we? Gosh, as much as I hate to say this, it might be easier to find a related species, do some experiments on it, and extrapolate from that.
Just my two cents.
Sushi and Sake,
John