Looking at the code p2 publishing code
(SiteXMLAction#getFeatureToCategoryMappings), it appears that if the
site.xml contains the associateSitesURL, then these URLs will be read and
set on the repository as references.
Using the "p2.category.site" property to categorize your build will probably
add the references to the internal build repo and not the final result repo
because I don't believe these references are mirrored between repos.