We are currently exploring the possibilities for an abstract route calculator/editor which is planned for TMS FNC Maps 2.0. Please note that this will take significant amount of resources and time and will not immediately be available.
I'm sure you've thought of this, but just in case you haven't. When the user re-routes the polyline, we'd need a way to extract the updated directions out of it.
That's the idea, each segment of the route will be separately available with step by step directions, toll costs, elevation data. With each new route, routes can be appended just by adding a new coordinate or by adding an address which will be automatically translated into a coordinate with reverse geocoding. All you will have to do is enter the necessary basic information, the calculator will do the rest. Please note though that this will take significant amount of development time. Additionally, the goal is to also create an editor that will use the calculator to create, change routes via interaction.