I love that I had this same idea today, search here and find an interesting discussion about it 
I see this would makes sense for a plugin initially, as a way to perform more actions to highlighted text on top of the current “quote reply”.
Like or share a portion of a message can be useful in some contexts, I see the value of having a non-visual-intrusive way (yeah maybe green is too much) to identify a portion of a long text that the community has identified as interesting or quoted a lot.
The way I see it, as a user I would be interested when reading a post to read comments related with a specific portion of the text without having to jump down and up, in context. But, maybe this is more annotations than just highlights.