The implementation of the "A to B repeat" feature and of the feature of "showing bookmarks (say, as subtitles)" I suggested recently could be based on a single generalized mechanism: the bookmarked moment of a video when passed during playback triggers a specified action.
In the case of "showing bookmarks as subtitles", the triggered action is "show the corresponding bookmark name".
In the case of "A to B repeat", the triggered action at bookmark B is "jump to A".
Perhaps, it could be useful something else.