Strictly my opinions/theory …
I think the Yemen/Veiled cham is geared towards getting a good amount of moisture via humidity. Where they live in the wild there is not a lot of rain but there is quite a bit of mist/fog coming from the ocean that they take in. The casque (and anatomy leading to its mouth), IMO seems like it’s built to catch the humidity and form droplets and it turn the droplets trickle down into their mouths but I also think they absorb moisture pretty well through their skin. The casque also seems like it can store fluid much like a camel does with its hump(s). So in other words, I dont think its normal for them to get "rain showers" every day via misting system or manual misting.
In my last apartment, it got VERY humid for a week, you could see it in the air when the light came through the window - humidity gauges actually said “Hi”, meaning that the humidity was too high to register. My veiled didn’t drink all week and dropped bright white/wet urates without any fecal matter almost every day. He was CLEARLY getting moisture from the air. As mentioned a million times before, chams get moisture not just through their mouth but also through their eyes, skin, nose and possibly the vent.
My veiled for the first year hardly ever drank in front of me – it was a bit stressful but I made sure I misted him twice day whether he liked it or not (important for eye cleaning), had a mist system installed since day one, used a humidifier when the humidity was low and when he was big enough I gave him a weekly shower. In that first year I never saw any signs of dehydration (so that took away some of my stress) and in time he accepted the manually misting and drank every day from it.
-roo