Sure, archetypical application of the ZS is/was for scenes "as found" with sheet film processed to match, but so what. ZS principles work just fine in any situation with a little bit of imagination and understanding. The whole system isn't required for success, even Adams offered advice for use with roll film and modern VC papers make print contrast adjustment a breeze for most shots.
Placement of one specific zone, like a face in a given zone, is about the easiest "part" of the ZS to cherry pick and quite useful.
Instead of shooting one or two sheets for a given situation, I just shoot a whole roll in each situation where ever specific development is needed. This is one reason I like to roll my own shorter rolls or shoot Medium Format.
There are lots of other options too.
The Zone System works great with artificially controlled scene contrast and subject placement, which is a very typical situation for many portraits.
In a studio every part of the scene is "zonable" so that they fall straight onto the paper where we want them.
In the wild the background can be "classically" zoned, exposure and development then chosen to match, just as Adams might have. Then the lighting for the sitter in front of that scene can be scrimmed or augmented to place them perfectly.
The intellectual magic of the ZS can be applied to any shot, it is simply that there is a real measurable "connection" of the zones in the scene to the zones that will print easily.