Something like an in-editor tutorial could be realized via an Editor plugin in the 2.1.beta branch of Godot. To walk through things when a game is run the plugin could set things up so the game could temporarily swap things in the script to custom nodetypes just prior to running (then back to the normal nodenames when the game exits).
Just a forewarning that the plugin code is highly bleeding edge and may not be completely working. In fact it is very likely to have a bug or two or three or more. Documentaion is also spotty and the documentation tutorial for plugins currently only covers two of the use cases (docking plugin and custom nodetype plugin).