Today I managed to get basic Universal Transcoder support working. Also Serenity will be using ExoPlayer under neath the covers for video playback. This should mean that you don't need to use an external player unless you want to with Serenity going forward. Serenity will always try to direct play the video if it determines it can't then it will fall back to transcoding. AVI files will always be transcoded to MKV files as ExoPlayer itself does not natively support the container format.
This is now implemented. I'm going to keep testing it for the next week or so locally to make sure that the basics are working but so far so go. I hope to have a new build out next weekend.