To be sure, I am not familiar with the full suite of CARs for maintenance. There would have to be some sort of trigger to progress to NDT using X Ray examination.
The way I read it, if there is not something already in the approved data for the particular aircraft, the operator is expected to pay a NDT specialist and CASR 21M Delegate to develop a program of periodic inspections.
Presumably they would set the intervals for inspection.
My gut feeling is CASA bit off more than they intended here. I doubt it was intended to include all aircraft, but I can't really see that you could claim the engine mount was a less important structure than e.g. the tail spring mount.