The case was "appealed" in Cassatie to the Hoge Raad (Supreme Court), in a procedure, which is only open to questions on application of (the principles of) law, but not anymore regarding the facts of the case. As I noted, the Dutch government has requested to become a party to those procedures, and was granted that right last month. Now the government has shed some more light on their motives to do so in answer to parliamentary questions of Van Nispen en Ulenbelt. The relevant answer reads (in my translation, with comments in red):
The Dutch State as on 22 May submitted its request to become a party to the proceedings with the Hoge Raad. It did so, because regarding the immunity from jurisdiction and execution attributed to EPO, the verdict of the The Hague Court of Appeal did not take into account sufficiently on one hand the international obligations resulting from the Protocol on Privileges and Immunities of the EPO and on the other hand the special character of EPO, which has a seat not only in the Netherlands, but also in different states. The Dutch state has therefore become party to the proceedings regarding
*The absence of jurisdiction of the Dutch Judge regarding EPO, because of its immunity as an international organisation with a seat in multiple countries [multiple countries argument; I have no idea if the number of seats of an international organization has ever mattered regarding immunity? The point of multiple countries was brought up by EPO of course].
*The scope of the verdict in relation to EPO's immunity from execution
The state has the duty to guarantee that international law is followed in the Netherlands. Violation of international law by the state and its organs leads to international liability of the Netherlands. That means that the State has to guarantee the immunity EPO has. The Netherlands should also ensure that its judges don't take jurisdiction, which they don't have according to international law. This means in the present case that the Dutch judge can not render a decision regarding an organisation over which it has no jurisdiction as a result of immunity. Besides that, the Dutch judge is not competent regarding subjects within the competence of other states. The decision of the court however is directed at the organisation as a whole, including its divisions in other states. For the Netherlands, as a host of EPO and many other international organisations, the international obligation to guarantee immunity is sufficiently important to become a party to the case on the side of this international organisation [after the legal arguments, that is a mostly opportunistic argument].
Well, the good thing of this is, that the arguments of the Court of Appeal are now being tested thoroughly at the Hoge Raad, and the State has every right to join the proceedings. But the strength with which the State dismisses a court decision in a case to which it is not a party is also troubling as it touches upon the balance of power between the judicial and the executive branches. It would have been a lot more respectful and appropriate if the State would have followed a different approach and said that "in the interest of development of law" it wants the Supreme Court to weigh the arguments to the maximum extent possible.
Will a decision of the Hoge Raad be the end of the business? In view of the importance of the ECHR case law regarding international organisations, an appeal there seems likely in the case of a loss of SUEPO in the Netherlands