Not really an answer… But this sounds a lot like the past beta feature called ‘leech detection’ which fell by the wayside, I think it was superseded by the development of the iOS and HTML5 apps. But anyway, it’s certainly possible and so maybe they’ll think about bringing it back?
See previous posts:
To take it further, this might actually kind of be related to this suggestion below; leech characters could be shown in the context of words they occur in: