At what temperature would a hotspot in a cement kiln become visible to the eye?

That depends to some extent on the human eye, which is variable. I can personally see them above 450 degrees C.

We are Clinker traders based in the UAE, and wish to start supply of coal to our clients in UAE. Wish to know at a very basic level - average consumption of coal per unit of cement produced. (one unit = 1 metric ton). The problem is I am getting a lot of varied opinions, and want to have accurate info before venturing into new trade.

At a basic level 0.1 tonnes of coal is used per tonne of cement produced.