Above the Clouds,
What Peter was saying is there is no regulation that requires you to be using a specific navigation aid when flying a non-AOC operation. Not that you do not require the specified instruments to be on board and working and the navaids to be working.
So there is no rule against flying an approach marked NDB while looking at your hand held GPS (although you must have a working NDB on board and the navaid must be working). Clearly you still need to follow the flight profile and if you fail to achieve that, you could reasonably by done for Careless and Reckless if you were using something clearly not up to the job. And I suspect a cheap GPS with hand entered coordinates might fit that criteria in the event of a crash that can be directly pinned on say an incorrectly entered waypoint.
The most common sensible application of this would be to fly and NDB procedure using a approach approved GPS/FMS. One would normally also of course load up the raw data as a cross check, but certainly I would be flying primarily with reference to the GPS data.