Are the four characteristics -- production, abiding, change and destruction -- empty, conceptual constructions?
Ultimately speaking, yes. I find this a difficult concept to explain so please bare with me.
Ultimately, Nirvana is the only thing that exists. Nirvana is both ever present and infinite. It is free of the four concepts of which you speak. For the sake of explanation, one could say Nirvana is the only thing that exists, and everything else we believe to exist is simply a conceptual construction or interpretation of Nirvana. Belief in those conceptual constructions is what causes the illusion of samsara to arise.
Here is a small example of how I would break this process down to explain. I know it doesn’t conform to the standard understandings of some Buddhist concepts like Nama and Rupa, but please just take this as a simplified explanation.
Imagine you see a tree. It looks as if it’s dying and will soon pass. You think how the four characteristics are present in the tree. But then you take a step back and realize that is not a tree. It is sight, which you have conceptualized to be a tree. You now think about how the four characteristics are present in sight. But then you take a step back and realize that it is not sight. It is Nirvana, which you have conceptualized to be sight. Upon observing Nirvana as the source of all conceptualization, conceptualization ends and the false self has nowhere to persist, and you become free.
Does this make production etc., perhaps even impermanence, an illusion, especially according to hua-yen Buddhism [as that's where I've seen these four characteristic enumerated]?
Yes. They are illusions, but they are helpful illusions. When studied, they can assist in guiding us towards liberation. But their usefulness comes to an end once they are observed to be illusions. This is why the Buddha says once liberation is reached, even the Dharma itself must be let go of. It no longer becomes helpful and only ties to to more conceptualization, which will pull you from the freedom you have just achieved.
Build the boat to cross the river, but don’t try to bring it with you when you reach the other side.
That being said, if you haven’t directly experienced Nirvana yet, don’t try to pretend you aren’t observing the four characteristics. It’s important not to ignore them and use them as tools to get a glimpse of the other side. Once you see Nirvana for the first time, the illusion will naturally break down, as will the understandings.
I hope this was helpful to you. I pray for nothing more than your success in your endeavors.