How to rotate / translate the whole animation (all key frames) globally (additive layer not in global coordinates)

Hi,

I have a number of motion capture animations that start with a T-Pose that's oriented in-line with one of the depth cameras used for motion capture, which determines the orientation of the global coordinates. These animations then include some exercise animations that are performed diagonally and offset from the origin, based on the location of an exercise mat, and maximising visibility from multiple cameras.

I have a number of these animations which I want to have a consistent origin and rotation that corresponds to the center of an exercise mat.

I've seen others asking similar questions and I have experimented with the possibility of using an additive layer or translating the x/y/z position key frames and converting the pelvis rotations to euler and translating the euler component key frames.

The problem with those options is that they are applying the rotations in the wrong coordinate space - the coordinates of the pelvis, but I'm simply trying to rotate everything in global coordinates.

I want to effectively rotate/translate the root bone, not add a local-coordinate rotation to the pelvis for every frame.

Currently I can manage the translation with an additive layer ok, but if I try and rotate everything then I end up tilting my whole animation instead of rotating everything in global/world coordinates.

This feels like it must be a common requirement to be able to rotate an animation to fixup its orientation so I'm hoping there's some trick I'm missing here?

Any help much appreciated!