Greetings Andrew,
I concur, of course, with the caveat that the structure is in fact "inferred" (as the superior structure for any new Funtional Location is defaulted based off the structure indicator), but can be explicitly altered. So the structure indicator is a guideline, not a constriction.
As you say, it is perfectly possible and sound to split a "hybrid" hierarchy between two or more Structure indicator (e.g. if "Z1" is an overlaying geographical structure, and "Z2" is a detailed technical; "Z2" FLs are then structured under "Z1" and may have totally different labelling convention). This may be particularly useful if you'd like the overarching structure, but it does not work well with the labelling/coding in the detailed structure.
Thank you for this clarification.