The only thing i have against a case is the risk adding a connection that would risk the waterproof part of this tablet. I will mostly be using this for kayaking so anything that increases the risk of failure when it comes to water seems like a very bad option.
As for using heat warmers to heat the device...remember you are going to have to fight the shell, the air inside the earl, the case, and the outside temp acting against your device.
The further away from the heat source your screen is the harder that heat source is going to have to work to fight the effects of cooling. In short the closer to the screen the easier and more efficient the heating will be.
If you have a nook with a e-ink screen do a test with a freezer and trying to warm the screen with your hand warmers...that will give you a good baseline of how effective the method will be to fighting cold against the screen.