That mostly works. The master page picks up the correct title to display, but the menu bar of the browser still shows the initial version of the title.
Is there any way to modify that as well or does that get set prior to any code on the report being executed?
I believe the reason this happens is because the title is read from the design by the viewer before it runs the report. The only way I know to work around this is to modify the viewer to add an id to the title tag and then add a Text element with client side script, to the report that locates the title element by this id and then use it to change on the fly.