A posted a enhancement request based on this at bugzilla [1].
In my view the easiest solution would be to add a tick box in the upload campaign for whether the image is part of this sub-competition or not. It would however also require that the placement of the wikitext on the image page could be controlled so that this template/category is not put in the description field.
I encountered a similar need during Wiki Loves Public Art where some lists were set up so that they could tell whether a FoP template should be added to the image or not.
The quick-and-dirty workaround is to simply have a field in the upload campaign with the following:
wikitext: "{{safesubst:#if:$1|WWI specific wikitext}}"
label: "Is this image part of the WWI sub-competition? Leave blank if not"
/André