- If the plaintiff's complaint alleges - in so many words - a violation of ius cogens, the immunity of the international organisation is off the table. The allegation alone is enough.
- Once immunity is off the table, the defendant will have to make an appearance. If they do, they can fight the chase in the usual way, if they do not, a default judgement will be entered, at least insofar as the ius cogens claims are concerned.
- Under Dutch law, a default judgement requires some sua sponte examination of the case by the court. Under art. 139 of the Code of Civil Procedure, a default judgement cannot be entered if the claim "appears" to the court to be "unlawful or unfounded". In cases against international organisations, the flip-side of point 1 above is that this test should have real substance: In the Srebrenica case, the plaintiffs talked about ius cogens a lot, but - taking all of the plaintiff's factual allegations as true - they did not legally amount to an actual claim of a violation of ius cogens. This should lead either to the UN's immunity being upheld, or to the request for a default judgement being denied.
This seems like a reasonably tidy way of setting it up, and it does a reasonable job of keeping substance and immunity separate without going so far as to giving the UN a pass for actual cases of genocide or - more likely - torture.