Overall I have been out of most of the skinning stuff, but I believe you can do whatever. Even with highway we wanted to try to keep the main functions in the corners for easy access but I personally take that as a usage recommendation vs a hard set rule.
As far as a reserved center area I am not sure. The few skin screens I have written have all been basic test data type ones to display sensors, gsp status, etc. Borte may be able to give you better answers there.