I am so happy because finnaly with this lastest version 188.8.131.52 i can stream 720p mkvs with external subtitles to the XBOX 360 and enjoy the CHAPTER feature which allows you to start the movie from the point you want.
It is possbile thanks to this solution which fix the transcoding 1.9 issue:
ANd ffmpeg, not mencoder, mencoder wont play CHAPTER feature.
But on this ffmpeg coding i cannot configure their Subtitles Font Size. I change them on the ASS font size but nothing happens, they keep so tiny.