Hello Steve,
Actually, I have meant, that you can import the masks from the external source using alpha-band or B&W images. Following this approach, Metashape wouldn't create new masks that are different from the existing shape of the external masks. But if you need to mask out the same area on multiple images (for example, some fixed element in the image corner, that moves together with the camera), then it can be done with the single mask applied to multiple cameras.
We are planning to publish some tutorials related to different masking approaches, not sure, though, that it would be videos - mot likely text with the screenshots and, maybe, animated illustrations.