This would be a very useful feature. As the original poster noted, rename is accomplished by closing a file, renaming it, and re-opening it. It is NOT the same as renaming an open file and keeping it open the entire time. So much unnecessary resistance to a very, very good idea. My document scanner saves PDF files as document (1), document (2), and I don't always get a chance to rename them according to their content at the moment I am scanning. I come back and open each one and then rename them. I can't tell exactly what they are from thumbnails. This feature would help tremendously.
Hope to see it sooner rather than later.
FWIW, I've written software in C++ under Windows and linux. Its just a matter of understanding what the users are asking for and not getting bogged down on why you don't think they should ask for it. The Mozilla team still doesn't appreciate why users want to right click and print web pages. I am grateful that the Tracker Software developers are not as hostile to users.