Can I vent a portable airco exhaust in the wall cavity?
Moist warm air  when it is exhausted behind enclosed walls (even slightly vented ones) have the potentially damaging effect of becoming condensate (water or moisture) if the temperature is cooler. Moisture is what mold spores need to thrive. Once they are established the colony can bloom and spread rampantly and unchecked out of sight. The health and safety hazards are well documented. A better solution for an exhaust for the portable air-conditioning unit to the outside is through the window. Open the window and measure the dimensions (L x W). Increase these numbers enough so that a piece of plywood overlaps  and fills the edges of the window opening. Cut a hole enough so that the exhaust vent can go through.