no way to do that w/o hacking or writing a custom app.
fortunately, there was a session at TC16 by ALEXIS GUINEBERTIERE on this very topic
Turning Any Dashboard into a Comment Database
To kick off this action-packed developer-oriented session, we will use a commenting app that lets end users annotate marks. You will see how to expand what Tableau can do with write-backs and mash-ups. We’ll dive into the nuts and bolts of hooking up a viz to a web application, and review some technical pitfalls when deploying to Server. Along the way we’ll also build a simple Ruby on Rails comments app in minutes that you can bring home and apply to your own workbooks.
and you can now watch it live!
click on the link below, login or register and watch!
note: this IS the link to the video. the page title is replacing the URL when i paste it in ...