Two alternative ways.
A possible elegant way to achieve this objective.
Starting point is here:
http://www.mediawiki.org/wiki/API:Categorymembers
with a default example at:
Even though it is not evident from the example, the "sortkey" attribute is what could help us. Basically, when one image is added to a category on MediaWiki, the syntax used on the page is: [[Category:Pleiades images]]
And the image (or page) will be listed on the category page according to the alphabetic sorting on the page name. However, this default behaviour can be tweaked by using a "sort key", as described in detail here: http://meta.wikimedia.org/wiki/Help:Category#Sort_order and one can imagine a syntax like
[[Category:Pleiades images|depicts=157802]] # a "random" place
The category page on Wikimedia Commons would not be tremendously useful, but the query API is totally awesome and it could be used to run a weekly/monthly run, caching image URLs locally and associating them to places.
Christian Consonni suggested using a tool that was developed for the Wiki Loves Monuments (WLM) photographic contest. It allows to retrieve all files that were given the same ID using a MediaWiki template. See e.g.
Fontana del Nettuno, Bologna - paese: Italia (it) - id: 037006698-MIBAC
The ID comes from a separate database (e.g. it is not on Wikipedia or other wiki) so it should work fine with Pleiades.
The easiest way to use this kind of tool would be to use a simple template like {{Pleiades|depicts|12345}}, that could also add a nice box and a link to the Pleiades place page (à la Flickr, even though retrieving the place name may be more complicated).
It is unclear whether the tool is available as open source code to be run customized and run outside of toolserver.