There are already feature requests for this. In the meantime, you can export the uncropped image to TIFF and use the retouch tool in the black areas on the TIFF. You can even create a larger canvas using certain image editors before doing further work in PhotoLab. (There’s a feature request for being able to increase the size of the canvas in PhotoLab, too.)
Do you know which raw processors currently are able to clone data to fill edge slivers without first exporting the image? PhotoLab can’t and neither can ON1 Photo Raw.