Well, there has to be an information key for it, so that it can be carried by the pipeline the same way as e.g. SPACING. But when it’s attached to the vtkImageData object, yes, an ivar makes sense.
So within the pipeline it can use a vtkInformationDoubleVectorKey with 16 values, and within vtkImageData it can be an ivar of type vtkMatrix4x4. That would make it easy to use. We could use a generic name, e.g. “MATRIX” for the information key and “Matrix” for the ivar.
I might recommend that the key be optional in the pipeline (to indicate unknown or identity matrix), even if the vtkMatrix4x4 was always present in the image.