> I want to refer to a registration in a registry.
But that's not the question you posed in the original post. You asked about URNs, and the URN registry imposes a taxonomy. You're wanting a registry of identifiers classified by country and type. That's a taxonomy.
Why depend on someone else's registry? Make your own, don't worry about taxonomy, and use it.
I want to refer to a registration in a registry. Objects can of course be regidtered many places for many reasons but that is not my concern here.
As long as I can serialize the tuple (registry-somone-made, id-in-that-registry) in a standard and readable way that would be a huge step forward