a device that you could program to display a rolling message, like a ticker tape of stock values:
Exactly what I was thinking also!
like an old piano that read punch cards.
This made me think of a model in the old 8888 “Expert Builder” idea book. (Picture Included below!!) It was a multi-function crane that could be “programmed” punch-card style. The cards are built from 6×16 plates with 1×4 gear racks arranges on them. By feeding the cards into the crane, different functions would be activated. It was basically Mindstorms 0.0.
The problem with this solution it that the cards were hand-fed into the device. If we use this principle, but build it on a technic tread conveyor, we could have a scrolling, repeating pattern (or message). Build an array of light bricks 5 tall by ?? and use slopes or pins attached to the tread to activate the lights as they pass.