Items on an altar?

Recently a church organization held a Human Rights protest in front of our church. Before the outside protest, they had some readings and such inside the church. It is my understanding that it was Not a Mass. During the time inside the church, they hung a poster board (with a scripture reading) from their protest on the front of the altar. Is this permitted to be hung from the front of an altar? I know that there are specific guidelines during a Mass, but I am not aware of this issue outside of Mass.

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