I'm not aware of this customization being done, but it could be configured.  We already keep a history of viewed articles in the "Recent interactions" section in the customer composite (and will be displayed on the "Recent cases" section with the Pega Customer Service 8.3 release).  Generally this article tracking/history has been sufficient for customers so other CSR's are aware of what articles were viewed during an interaction/service case when looking at an historical view of an interaction.