Hi Peter, thank you for your answer,
so, there is no way I can read the sensor values without modifying the engine’s source code? FOculusRiftHMD is in the private part of the plugin, so it’s not visible when I don’t compile the engine myself.
This is not a big problem now, but it might become one in the future 
My way of solving the problem now would be the following:
Add the following three lines to IHeadMountedDisplay:
virtual FVector Get{Gyroscope,Accelerometer,Magnetometer}Readings const { return FVector(0.f,0.f,0.f);}
The HMDs implementing IHeadMountedDisplay can then decide to override this function. If not, the default value (0,0,0) is returned.
I would then override the functions in OculusRiftHMD.h and return the sensor readings.
I hope this is what you meant 
Is this a clean enough way to do or am I overseeing something?
Regards,
Sebastian