In reply to Dheorl:
> Does anyone actually know the answer to my question?
It depends... Mostly on the condition of the snow.
If the snow is nice and cold and dry, and you have a decent amount on insulation on your legs, then you can sit in a snow seat for a long time, and you won't get a wet arse or ass if you're wearing non-membrane softshell trousers (such as the Simond Alpinism). Snow is mostly air, and is a good insulator. You sitting on it won't melt it much. If your arse feels cold, it means your body heat is leaking out of it, which probably means you're sitting in something pretty damp, and you arse is likely to get wet. Either that or you haven't got enough insulation under your trousers. Some Paramo trousers and salopettes (which are essentially a type of soft shell) come with a foam pad that can be inserted in the seat, to provide that insulation.
On the other hand, if the snow is warm and slushy, you are essentially sitting in a puddle of water. In which case, water will be forced through the non-membraned soft shell, and you will get a wet arse. In these conditions, you need a waterproof seat to the trousers, as a minimum, or full hard shell trousers.