Well, for one, if you have a video to save, move it into another directory. You need to render it to AVI before Valve updates and breaks your demos, anyway.
However, you should have no trouble with this. Just make your automatic cfg have a line of exec timestampedvideo and launch from a script that will make a new timestampedvideo.cfg file that contains a record command followed by a string that represents the date, or run-instance number, whatever you like, before launching the game. (You can get your launch string from a shortcut icon.)