Well, the power supply won't be completed exposed to the elements. I will place it, along with another one which I use to power up all my other accessories, inside of a fishing box. When operating, these two power supply should produce enough heat to keep the box warm enough to avoid dew or frost around them.
I have seen many electronic components such as laptops, cameras, USB Hubs, autofocusers where their manufacturer state that the lowest operating temperature is zero degree celsius and yet we have been able to operate them below this value without problems. I guess, the real danger is to try to operate them if they are wet due to dew formation but the low temperature itself shouldn't be a problem.
Anyway, we will see how it will go!