From plotly's API, the function I think you want is add_segments()
If you check Carson's plotly book, there's a section where he talks about how you can extend ggplotly() to accomplish this--just scroll down a bit and you'll see a section where ggplotly() pipes into an add_function() call where you can use add_segments()--hope that helps!
edit: I think that it's worth the time to learn how plotly's API works--it can be really rewarding!