Para b makes it clear that a private aircraft registered in a 3rd country may operate under an approval from the state of registry.
Indeed, but this is additional to the GPS approach approval which an N-reg will typically have, and I don't see the FAA setting up a "special procedure for European owners" for generating these approvals.
Even easier, and what the FAA did yonks ago was create "Overlay" approaches. In other words they said "Here's an old NDB/VOR approach into XYZ, we realise that NDB's are ****e, and GPS is far mor accurate and so now you can officially fly it using GPS as primary reference". The Approach plate is labled something like "VOR or GPS RWY 30".
True, and this is something which Jepp do in-house, but the overlays can be done only where a conventional IAP exists. An IFR GPS will contain such overlays, though (for me, UK, KLN94) the representation is often pretty basic and I find it a bit confusing, and it is easier to fly the published IAP using the OBS mode of the GPS.
Of course anyone with an IFR certified GPS and any sense of self preservation will activate the XYZ NDB approach on the GPS and fly it using the GPS, possibly backing it up with the ADF, even in Euroland.
Exactly.
This is what airlines do. They fly NDB approaches (to various Greek islands, etc) using the FMS (which in reality means INS with DME/DME and, on modern planes, GPS, corrections). The procedure varies according to their AOC manual. All need the navaid to be not notamed INOP. Some need it to be tuned and idented. Some need the ADF to be checked at the top of descent (I believe this is a UK CAA one). One I heard of requires the ADF to be within 5 degrees all the way down, which is silly as it would mean you will never get into any coastal airport whose final approach track is not perpendicular to the coast (unless you ignore the ADF, obviously
).