Having practiced ZZ in some other Qigong systems, if we are basically talking about postures, my guess is in the "Sitting Meditations" part of SFQ, this works fine.
Chunyi Lin says that you can do these meditations "Sitting, standing or lying down". Well, once we are sitting, standing or lying down, the question becomes, how do we sit, stand or lying down? Where do we put our hands? What about our legs?
Since zhan zhuang is basically standing in a posture, using Small Universe or Self-Concentration in one or more of these postures during our meditations, should work well.
Coincidentally, I have been doing just that for the last week or so. I also do them sitting and lying down. It depends on my day.
Since I am not yet able to do the full lotus posture while sitting of the three options, sitting, standing or lying down, I have felt the most benefit from standing. While standing, I hold the energy ball or one of the other very simple positions of the hands that are featured in ZZ. In my limited experience, there is nothing in a simple posture that by itself is a "different energy system".
If one practices a different form of breathing during the posture, then that would be a different system. But then one would not be doing Small Universe for example.