I couldn't find any best-practices for converting from ARF to MP4 - particularly removing background bar due to the strict 1024x768 size imposed by the ARF converter. This is the highest quality process, I was able to come up with
- Windows machine with WebEx ARF Player
- Windows, Linux or Mac machine with ffmpeg
- Record session
- Edit recording in WebEx to set start and end times
- My Event Recordings > Recording Actions dropdown > Modify