Although, another space to consider would be the larger landscape advert projections on the actual platforms.
I have found the best source of information to be from http://www.transportmedia.co.uk/london-tube-underground-advertising
Variations of screen advertising on the tube.
My original idea of the digital escalator panels would work quite nicely, they are short and require no sound.
Although this website was helpful, it didn't offer any information in terms of timing and dimensions of the ad spaces. It also specifically outlines on the website that they do not want to be contacted by students wanting information for research... gutted.
This video gives me an idea of different ways I could use the ad space, like spreading the animation across more than one panel. It also gives me a better idea of how long the animations should last for.
I have found an article, which includes information saying that the time slots sold for the escalator ads are 5 or 10 seconds.