Sounds to me like the sanctioning body has not addressed this
in a non interpretable way or the techs are not doing their jobs inspecting.
Could it be considered a throttle stop, maybe set up
like a fast idle cam on a choke system.
I could see the idle solenoid being considered not legal , but not a throttle stop. If I was in the class and I knew another racer had the idle solenoid, I would not say a word. Mainly because I would be using something mechanical to do the same thing.
Difference being that one is debatable as illegal, and the other is black and white not illegal.
I would say that it would depend on the track and who was doing tech, as to if the solenoid is illegal. If you had that setup you most likely would run into problems if you raced at multiple tracks.