To my knowledge, it's been invoked in at least one other case involving gambling laws in the United States. I believe it was Antigua that challenged a number of restrictions on cross-border gambling. The U.S. sought to justify those restrictions under the public morals defence. There is a fair amount of WTO jurisprudence under GATT article XX, the general exceptions, generally and specifically in relation to this idea of necessity.
In looking at the general exceptions, there are various components to the test. The first one is that you have to establish that the measure is there to protect something. Is it protecting public safety, protecting public morals, protecting human health, protecting animal welfare, or protecting the environment? There's this notion of protection.
The second step is this idea of necessity. Is the measure necessary in order to achieve that objective of protecting, in this case, public morals?
Then you have this third step, which falls within something called the chapeau where, nonetheless, one has to determine whether the measure has been applied arbitrarily or in an unjustified manner.
There's a significant amount of jurisprudence around the general exception test itself.