Set Poster Frame

My Movie and TV collection on my media server is starting to look good, but heaps of the movies and programs start with a black frame. By default, this is the Poster Frame when the movie is previewed on a computer. Quicktime Pro has the ability to set the Poster Frame, but cannot just add this information to the file, it must re-save the file. With even the smallest of my files (The Simpsons episodes), these files are in excess of 100Mb, which takes a long time to re-save. Particularly over a network, and with the NSLU2 using a USB Drive. I want a way of just telling the movie file that it needs to use a particular frame as it’s Poster Frame, and not have to go through the whole rigamarole of re-encoding the movie. I don’t mind if it runs on Win32 or MacOS, as long as it does the job quickly and easily. Even if it’s a CLI program that requires the frame number: I’m sure I could come up with a way to find that out…