What role does zen or zazen play in Tendai? I would like to know how Tendai or indeed Tiantai, views its use of zazen or zen.

I would provisionally assume it serves a different role than it does or is in Soto and Rinzai and contemporary Chan.
The Tendai school has its own system of meditation based on the writings of the founder of the school, the great Chinese monk Zhiyi, who wrote a classic meditation book called the Mohe Zhiguan which means The Great Shamatha Vipashyana. This meditation style in Japanese is simply called Shikan, which is the Japanese pronunciation of the last half of the name of the book meaning Shamatha Vipashyana.

I don't know too much about it but it is a different system of meditation than Soto Shikantaza or Rinzai Koan practice. I think that it is a more structured and explicit system of meditation but I don't have much direct knowledge on this point.