Discovery Green has its own offices. The administration of the park is who decide about advertising and promotion of a brand or company. Is not with the City of Houston. The office is next ti the fountain.
I would definitely check with the City of Houston. Considering that businesses pay to have their goods and services displayed in public spaces, I would say, "Yes, it is illegal or you will be fined / asked to stop if you do it.,"
I would assume so. It might hinge on what you mean by promote.
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