Currently, there is no implementation for calculating the angle of non-standard terrain automatically. You will either have to construct the tube out of standard masks, or wait until I've implemented a method in a new build.
(If you choose the former, you have options. You can create a child of NonSolid with a reaction_script that puts the player into player_state_rolling by force, and place it at either end of the tube. You will then need to prevent the player from jumping out of the state somehow or uncurling from moving too slowly. Alternatively, you can make your tube object a child of objPath with a custom reaction_script that sets the player to a path matching your zoom tube; you'd also set the player to a new state based on player_state_path, where you'd handle acceleration and going backwards, and in the End of Path event, you'd put the player into player_state_rolling at a speed relative to the path_xspeed and path_yspeed.)