I have some new fresh info
I have just finished to talk on the phone with Domenico, the producer of Amanero USB boards.
We had a very friendly chat, also because he comes from a town 60Km away from me.
He explained that it already exists a firmware capable of DSD512 under Linux, but he just offered the tools to designers and at the moment it is up to each one of them to patch OS in order to work in DSD natively.
I suggested to contact ALSA maintainer to make such patch public as part of Linux next release.
He will do so and keep me informed.
He also told me about another board in preparation, but that's another story ....