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You could you $(CurrentUserOrganizationalUnit()) or $(CurrentUserOrganizationalUnitName()) as a constraint on the Self Service Role for the FRS_Knowledge object. You then need to add an OrgUnit field to the Knowledge item to use with the filter. The main problem is not duplicating knowledge articles as each one I think would have to sit with a single OU. Not something I have really played with in this configuration but would work.
Thanks very much for the feedback. That is actually what I did as I didn't see any other solution. As you rightly mentioned, one article can only sit with one Org Unit.