Have you looked at modules that may provide similar functionality like:

1. https://drupal.org/project/annotator
2. https://drupal.org/project/annotate

Other links:

1. https://groups.drupal.org/node/72748
2. https://github.com/patrickmj/Open-Review

They may not solve the problem you have but they may have code solutions that can help you go down the correct track

Have you looked at modules that may provide similar functionality like:

1. https://drupal.org/project/annotator
2. https://drupal.org/project/annotate

Other links:

1. https://groups.drupal.org/node/72748
2. https://github.com/patrickmj/Open-Review
3. https://groups.drupal.org/node/10875

They may not solve the problem you have but they may have code solutions that can help you go down the correct track