I got this idea to create a servlet filter, that would inspect the thread-local store for the thread currently processing the request and log any thread-local references that exist before the request is dispatched down the chain and also when it comes back. Such a servlet could be packaged as a Confluence Servlet Filter Plugin, so that it is convenient to develop and deploy it.
E. Lacic, D. Kowald, P. Seitlinger, C. Trattner, and D. Parra. Hypertext 2014 Extended Proceedings Late-breaking Results, Doctoral Consortium and Workshop Proceedings of the 25th ACM Hypertext and Social Media Conference (Hypertext 2014) Santiago, Chile, September 1-4, 2014., volume 1210 of CEUR Workshop Proceedings, CEUR-WS.org, (2014)