If I recall correctly, Imperial Arts actually did attempt to film an evocation in which someone else was in the circle with him. Before the entity made any kind of appearance, the individual in the circle without the robe, girdle, etc., became violently ill and the evocation was terminated. His preliminary conclusion, if I remember, was that the purpose of the various garments and protections are both to prevent the spirit from doing harm as well as deceiving the exorcist. This actually in certain ways suggests that the spirits are perhaps always present, but only visible when they are constrained, not so much to appear, as to not make themselves invisible. This is in line with some of the other observations he has made regarding the nature of the spirits' powers to bestow certain abilities and powers - namely, that rather than do something to the exorcist to enable these abilities, the spirits are constrained - exorcised - such that their influence no longer prevents the exorcist from accessing a god given ability that was already available but obscured by the spirits.
This being said, and assuming I interpret the information correctly, and assuming it of course reflects accurate accounts, my conclusion would be that a spirit does not have to be filmed if it does not want to, and filming it could potentially be dangerous as well.
On the other hand, it could also be that the construction fo the circle, the ritual involved, etc., is intended to create a kind of spiritual holographic 'screen' upon which the scene external to the circle plays out - in other words, it is a psychic event only possible to observe from inside the circle, and only by the exorcist or compatriots who are living beings with a consciousness to operate upon.
The world is complicated - that which makes it up is elegantly simplistic, but infinitely versatile.