If you put the devboards in an enclosure with vents on opposite sides and a small exhaust fan on one vent, you will get much better longevity, as that keeps air moving across the hot parts. I don't know if it's a better solution than the chromebooks though.
In high school I tried building a PC into a refrigerator, and the results weren't pretty. The fridge could not keep up with the heat being generated by the CPU and over time it got too hot and shut down. The electric bill was also quite expensive, keeping a fridge running full blast 24/7.
Maybe hacking up the fridge to have the Freon running right over top of the CPU, but I'd rather take my chances tearing apart a Chromebook than tearing apart a fridge.
You cannot put heat generating devices into refrigerators. The thermal load they are designed to handle is quite small. They cannot handle a continuous internal heat source.
There are some definite advantages; the thermal resistence from components to mineral oil is something like 4x lower than to air. However, you need to be careful as I've seen mineral oil travel up cables and over the top of the aquarium (was the worst with IDE cables, I expect it would be bad with these fancy nylon or fabric wrapped cables you see today as well).
