The disadvantage of that method is:
a) it is incredibly geeky. I haven't got it to work properly - let alone I could explain it easily to someone who learned to photograph in the analog times
b) it is a rather binary method - it is in the category or out. That doesn't allow any degrees. 

It is a possible fallback scenario, so thanks for pointing it out, but I don't think it is exactly what we're looking for. 

Lodewijk

2012/6/22 Federico Leva (Nemo) <nemowiki@gmail.com>
Bas vb, 21/06/2012 19:59:

Last year I scrolled to the categories (looking 200 pictures in 10
seconds), every picture that I liked or jumped out was then opened in a
new tab by me, and if I liked the picture I copied the link and added it
to a page with wikistyle pictures on big size (700px) this page I
watched in preview edit mode and this way narrowed down the selection.

I think a basic but strong to tool to speed this up very much is to skip
the copy the link and place it somewhere else part of that and make that
a simple mouseclick.

Such a tool exists already, it's cat-a-lot with a (temporary) sub- or parent category. You said it shouldn't involve saving on Commons, but I don't think transparency here is a problem: rather, given the number of images and the high chances of mistakes, it could be an asset; more likely, nobody will bother to follow closely.

Nemo