I've been meaning to do this for a while, and I finally got to it today. You can now add new route points between existing points instead of only at the end of the route. After creating a route, if you click on the route itself, it will add a point at that location. Be a little careful, though - if you click on a campsite or portage instead of the route, it will add it to the end of the route instead of in between the two points. 

Please let me know if you run into any problems using this feature.