Thks Dimmy for this interesting suggestion. I think that it is not easy to hack the current code, but as I am worked on the next release, may be I could integrate this option.
From my point of view, the selection of a "category" could be managed as the searchWordList parameter. You
could see this option runs here
For instance by defining a parameter &categories=`area1,area2,area3,area4` where when linked with &parents=`12,34,67,78` the selection of "area3" restrict the search to the subset provided by the parent #67. And a selection of "area2" and "area4" restrict the search to &parents=`34,78`
And the same thing if you use the &documents instead of &parents.
But have you any idea of how I could manage this new parameter if I get a number of categories smaller than the entries in the &parents or in the &documents ? May be by create an category name by "category#id"
Thanks for your comment about this proposal