I did the ATPL law exam about a year or two ago (foreign conversion). I remember one of the questions, as follows (my phraseology):
- You are tracking 200 to pass overhead an airfield. The airfield has an NDB and a localiser. The localiser final approach course is also 200. What should you use for tracking guidance, for the final 20nm? The answer of course is the LOC, regardless of what navaid you've been using up until now. I got that one right!
It is of course entirely inappropriate for the enroute waypoint context, but it's in the AIP regardless.
It seems clear to me that it's contrary to the AIP to conduct an NDB approach in IMC if there is any other type of approach available for that runway.
Applying non-CASA-approved common sense, I think we all know what they
mean by the rule, and it falls into the category of "good airmanship". On a fine day for training, do whatever you like. If the differences between navaids is going to be critical for airspace safety, use the most accurate one.