You obviously are very passionate about the needs of 
ECV users. It’s good that people have advocates. But I would be shocked if Disney made it possible to exit - and therefore enter (with help if necessary) - two of its most popular rides undetected by cast members.
Fortunately for both Webslingers and Radiator Springs cast members are very easy to find. If you do need to exit the line then simply double back to the entrance (should be on the way to the bathroom) and find them, letting them know you had to leave and asking what the procedure will be upon your return.