[FFmpeg-user] HLS - Insert timed metadata or timed event into HLS

[HyVu2 Mai]

Is there a way to insert in a timed metadata or timed event into HLS content? For example, at 5 seconds into the movie, the player (Android exoplayer) can receive a timed metadata or timed event, so that the player can offer the user to skip the introduction. 

So far, I used ffmpeg to take an MP4 movie and create an HLS format with fragmented mp4 segment.
ffmpeg -y \
  -i Allied.mp4 \
  -force_key_frames "expr:gte(t,n_forced*2)" \
  -s 1280x720 \
  -c:v libx264 -b:v 1500k \
  -c:a copy \
  -hls_flags single_file \
  -hls_time 6 \
  -hls_playlist_type vod \
  -hls_segment_type fmp4 \
  -hls_segment_filename "single_file.m4s" \
  prog_index.m3u8
But now I need to insert some timed metadata or timed event in to it. What options does ffmpeg have that can support this?