The UK has loads of these "secret" approaches.
Only a specially CAA approved operator is allowed to fly them.
In many cases, nobody is allowed to fly them anymore but they are still unpublished.
It has been alleged that this is because the operator originally paid for having them designed so it is his intellectual property, and if the details leaked out, his competitors would obtain an unfair commercial advantage.
In general, you can find details of these from the locals. Local instructors often use them to teach the IMC Rating.
In any case, one would fly an NDB approach using an IFR GPS, using the OBS mode, checking the ADF at the TOD for example.