I know this is an old thread, but I stumbled on it and it made me think of my grandfather tonight. Years ago he had a device that was sold commercially that hooked up to the throttle and held it at a constant position. A lever set the position to the desired speed and you could move the lever up or down to tweak the speed. There was one safety feature that would disengage the thing if you stepped on the brake. It had a solenoid that released the mechanism. My guess is that something like like this would accomplish what you are talking about. It would drive all the other motorists crazy, but then it's not much different than me staying in leanburn in traffic. My grandfather's device was chrome and kind of ugly and probably only made to work for cars in the 50s and 60s that had big dashboards, etc., but for it's time it was a ecomoder-type device. All I have left of that device today is the solenoid
