Custom tracks are now setup and working! We took a different route then we planned with this. Instead of programming a huge editor from scratch, the tools that we use for the offical tracks have been ported into a Unity package and touched up to be more user friendly. These tools consist of various track tools and the game's lightmapper.
What this means is that after you have created a track in the track creator, you export it out and import it into the Unity Editor. From there you use the provided tools to configure your track, a wizard is provided so you can do all of this with a single button click. After that you build an asset bundle then the game can read and load that.
These tools are already available to Patreons on the developer branch. If you would like to learn how these tools work ahead of time, documentation is going up right now: http://ballisticng.wikia.com/wiki/Ballistic_Unity_Tools
Workshop
Custom tracks will be uploadable to the workshop. The current proposed plan is to have an archive file that contains the asset bundle and a track floor for the menu. A manager will be implemented into the game so you can mange custom tracks, since the process of importing a custom track is different to that of importing a custom ship it won't be integratable into an editor.
Why use Unity?
By letting people set up their tracks in Unity, we are not only saving ourselves an unrealistic development time but we are also providing people with all the power they possibly need when making their tracks.
Unity and all of it's features are completely free, so anyone can install it and use its full potential. We are looking forward to what people do with this in 0.7! And to end off this announcement, here is a screenshot of the tools in the Unity Editor.