I don't know much about video and Flash, but a few days ago I wanted a song from a website that appeared to be embedded in a Flash file. I looked at the source, downloaded the .swf, opened in Flash Pro. Flash Pro then gave me an error saying it couldn't find the .mp3 file and gave me a path. I then reconstructed the http:// path opened up Terminal and used the curl command to download the .mp3 file.
Well the point is that I was able to access media the content creator probably didn't want me to access, or else they would have given a download link on the page.
If someone wants the video file, they will get it.
You might want to look into having the web-server look at where the request for the file is originating from. If it comes from your domain through the Flash file, then permit access, otherwise, issue a 403 forbidden error.
-w0ngbr4d