""" [Peter van Dijck]
I wrote about the concept of "nested facets" here:
http://poorbuthappy.com/ease/archives/002793.html
[...] I'm looking to see if this concept of nested facets has
been described/discussed elsewhere (in the LIS community?),
probably using other terms...
"""

Peter,

something very similar to your idea exists indeed in general
bibliographic classifications, both faceted and non-faceted:
it is usually called _common subdivisions_, or
_common auxiliaries_, or even _common isolates_.

For example, Dewey Decimal Classification has tables
of common subdivisions for places, historical periods,
person categories, languages, and so on, which can be attached
to many different classes within different disciplines
(see EG <http://www-class.unl.edu/curr921b/contents5.htm>):

-782 Nebraska

3729 Elementary education in particular regions
3729782 Elementary education in Nebraska

91(0) Geography
91782 Geography of Nebraska

Please notice that DDC is not a faceted classification:
common subdivisions are not a feature exclusive of FCs,
hence a librarian would not call them "facets".
I agree, however, that they are used in a way similar
to facets, and maybe can be seen as a first step towards
the idea of FC, namely of concept combinability, which
was developed later (since 1920s) in library science.

Faceted classification systems, like BC2, have both common
subdivisions and facets. The main difference is that facets
are limited to a specific discipline -- EG "habitat" is only
a facet within zoology -- while common subdivisions can
be applied across disciplines, as in the example above
with education and geography.

Hope this helps...