The use of going to refer to future events suggests a very strong association with the presen
t. The time is not important, it is later than now,
but the attitude is that the event depends on something in the present situation that we know about. Going is mainly used to refer to our plans and intentions or to make predictions based on present evidence.http://www.ef.com/english-resources/eng ... ure-going/
So, my thinking is that it applies to the event at hand, the author can safely state what is going to happen, on the grounds of something that he knows about (but that he chimes in to inform "at the time" of the conversation between the characters).
In fact "was going on display" there´s no "to" along with "going" but to me the reasoning holds.
Anyway, outside that point there are certain expressions that use "go on [what you have...here meaning activity]", so "going on display" would mean "to be in display" (em exibição). It´s a fact that "to" don´t go well with some verbs and this seems like being one case.
All said "on display" is largely used to say mean that some piece of art that is going to be part of some exhibition, in the immediate future or happening at the moment of the speaking/giving the information.