Strictly speaking, the "6 month" rule is only available to temporary visitors to a particular country (as opposed to those taking up permenant residence) and is fairly evenly applied throughout the EU under the provisions of the EU 6th Directive on VAT and the Single Market legislation.
However, none of this statute covers local taxation (yet!!) or vehicle licencing legislation. This is why Greece (and for that matter the UK) can set its own rules and charges for the registration of privately "imported" motor vehicles. So far as can ascertain, there is nothing to prevent any country refusing to register a particular class or age of vehicle, to charge whatever fees or taxes it likes or to restrict the use of such vehicles (witness the restriction placed on classic cars in France, where they canonly be used on organised club runs etc!). On this basis, I doubt whether ther would be any recourse to the European Court Of Justice (ECJ) in such matters as I feel that the subject is beyond the remit of the Court.
Whether, on the other hand, there would be any recourse to the European Court of Human Rights, I don't know. Maybe someone has deep enough pockets to find out!