HC-
First, let me admit that the only detailed knowledge I have of the EC225 MGB design in particular is that I have gleaned from the public domain. However, I am an engineer by trade with a fair amount of rotorcraft drivetrain design experience.
As for your question regarding the impact on MGB "integrity" resulting from adding non-contacting instrumentation such as shaft radial displacement probes or shaft angular position sensors, there would be no real impact on "integrity". These sensors are solid state devices, and are thus extremely reliable and rugged. However, I believe even the best current MGB HUMS typically only employ sensors for chip detection and vibration monitoring with regards to detecting structural failure of rotating components such as gears, bearings or shafts. And there is nothing similar to the approach I showed currently in widespread use.
The outputs of turboshaft engines commonly include a "torquemeter" that measures the speed and torque transmitted through the output shaft. The torquemeter often consists of a pair of angular position sensors (such as Hall effect devices) and trigger wheels at opposite ends of the output shaft, which measure the torsional deflection of the output shaft under load. The engine HUMS records the accumulated shaft rotations and torque moment applied during each such event.
Lastly, even the most basic MGB HUMS utilize instrumentation that would be considered "invasive". There are chip detectors in the lube oil return circuit, there are pressure and temp sensors in the lube oil pressure circuit, there is a differential pressure sensor in the lube oil filter housing, there is a level sensor in the lube oil reservoir, etc.
Regards,
riff_raff