Reverse is possible by operating on streams using Reverse() method:

IMediaInfo inputFile = await FFmpeg.GetMediaInfo(Resources.MkvWithAudio);
string outputPath = Path.ChangeExtension(Path.GetTempFileName(), ".mp4");

IVideoStream videoStream = inputFile.VideoStreams.First()
    .SetCodec(VideoCodec.H264)
    .Reverse();

IConversionResult conversionResult = await FFmpeg.Conversions.New()
    .AddStream(videoStream)
    .SetOutput(outputPath)
    .Start();

In given example output video file will have only one stream - reversed first video stream from source file.

Use Reverse() method is possible on IAudioStream and IVideoStream.