I think the problem is not as difficult as you have described.
If we look at http://www.wikidata.org/wiki/Wikidata:Notability
will see that each wikimedia commons page can have a corresponding item.
The comment that "a sitelink to a category page in Wikimedia Commons is
*not* allowed on main article items" means that Commons Category pages
should link to Category items and not to items linked to wikipedia
articles. It does not mean items linked to Commons Categories are not
allowed. I fact I believe nearly every Commons Category has a corresponding
wikidata category item.
Notability Criterion 3. reads "(An item is acceptable if) It fulfills *some
structural need*, for example: it is needed to make statements made in
other items more useful.". I believe that this allows the creation of items
for institutions, photographers, books etc as required to describe Commons
files. Considering the two examples you identified:
Category:Images released by British Library Images Online
Each of these images can have the statement 'Source:British Library Images
Online'. This statement requires a CommonsData Property "Source" and a
wikidata item "British Library Images Online". As this wikidata item is
needed to complete this statement therefore it meets wikidata notability 3.
Category:Metropolitan Improvements (1828) Thomas Hosmer Shepherd
Again wikidata items can be created for the book "Metropolitan
Improvements" and for the author "Thomas Hosmer Shepherd" and for the
publisher (if known). All of these are clearly all considered as notable
. These wikidata items can
then be linked to from statements in CommonsData describing each of the
Note that this all works without needing to link to the Category Qitems.
In practice this means that if a Commons file is in a certain category then
we can know that certain statements will apply to that file. Later,
eventually, we can find those files by searching for files to which those
statements apply and ignore the categorisation since all the information
inherent in membership of that Category has been included in the form of
statements. We do not need a "container for structured information
for structured information associated with each commonscat". This
structured information can just be included in CommonsData, without any
Eventually, when the information inherent in the categorisation system has
been translated into structured data, and the query system is a lot more
useful than today, and the Categories based on idiosyncratic selection
criteria have been transitioned into Galleries where they should have been
all along then Categories may no longer be needed.
But perhaps we will keep them anyway.