Thought of? Yes. Made? Maybe.
Do you know why there is such a thing as an idle timeout in the first place?
It's to keep traffic down. A player that does not play still needs to send and receive data from the server.
If every player stayed connected all the time, there is no way the server can handle it, even with the best internet connection available.
That being said, we addon authors try to improve the game for our own benefit and that of others. If something like an anti idle addon where to be made and became widely popular, ZOS would be forced to do something against it and either ask us to take it down, or lock the UI in some way, as it would threaten the continued service of the game.