It would be useful to exclude some objects from reflections. For instance, if there is a scene with a car model and the environment should be reflected in the car windows, the car model itself is not desirable to be part of the reflection. Excluding it from reflection generator would increase the performance.

I see 2 ways to implement it:
1) Add castsReflections property to Model QML Type
2) Add a list property to ReflectionProbe with all Models which should be taken into account in reflection generator. This one is preferable in my opinion as it would make it possible to define different lists for different reflection probes.