The discussion eventually turned on Art 39(1)II Dir 2014/24, which states: "Contracting authorities shall not use the means provided in this Article [ie mechanisms of cross-border collaborative procurement] for the purpose of avoiding the application of mandatory public law provisions in conformity with Union law to which they are subject in their Member State."
This can be seen as an anti-circumvention clause aimed at ensuring that contracting authorities do not seek to disapply mandatory domestic rules by 'escaping' their jurisdiction through international collaboration--and, consequently, as a rule aimed at preserving the competential split between Member States and the EU in an area that arguably exceeds the procurement remit and goes to the core of the principle of national procedural and organisational autonomy.
However, participants in the seminar raised the point that it can also be seen as a 'Trojan horse' indicating further legal integration (and further regulation of these mechanisms in a future 6th iteration or generation of EU public procurement Directives) through the test of 'EU compatibility' of domestic mandatory public law provisions. I find this a very interesting thought, which is worth exploring in more depth. For now, I can only offer a few initial reflections.
From that perspective of 'EU law tests creeping into mandatory domestic public law requirements', and taking the example of free movement of goods, the question would be whether Art 39(1)II Dir 2014/24 does no more than recreate the mechanism of Art 36 TFEU--ie bring to the area of public procurement a 'public policy' (+ proportionality) test that mandatory public law requirements need to meet in order to justify the restriction on free movement that derives from preventing contracting authorities from resorting to enabling provisions for collaborative cross-border procurement. Or, on the contrary, whether it creates a separate test of 'EU law compatibility' that can actually go beyond what could be defended by Member States from a free movement of goods perspective by forcing an interpretation based on the effet utile of the rules in Dir 2014/24 itself--which would, almost by definition, result in more limited scope for absolute restrictions on the possibility to engage in collaborative cross-border joint procurement.
Either way, and having in mind recent cases on 'public policy' justifications for restrictions on free movement of goods, such as the DocMorris 2 case, it seems plausible that Art 39(1)II Dir 2014/24 may effectively be used in the future to demolish traditional public law requirements applicable to public procurement (such as subjection to domestic public contract law, language requirements, etc) on the basis that they disproportionately (or absolutely) restrict the possibility to engage in collaborative cross-border procurement.
For the purposes of the emergence of trans-EU public law, this would be a clear lever for the transformation of Member States' domestic public law requirements applicable to procurement activities, not least because internal market-type analysis would start being applied to public purchasing arrangements and their regulation in a different and possibly more stringent fashion.
So this is an area where I plan to keep an eye in the future and where I would appreciate input concerning any cases that may be developing at domestic level in the Member States. Either now or in the future.
