I've come across the following site -
http://schnabelmusicfoundation.com It provides a great deal of information about the Schnabels.
It contains a fascinating piece about Karl Ulrich Schnabel written by his grandson, the late Claude Mottier. It was entitled 'Karl Ulrich Schnabel's Approach to Expression'. He delivered it a symposium about Artur Schnabel which took place in Berlin in September 2001.
Hope this is of interest.
Regards, David Mendes