Another solution (possibly works in 2.8, too):
* Start the Crop tool, and tick the "Allow growing" , this uncovers the "Fill with" selector that you set to "Transparency".
* In your image window, make the crop shape outside the canvas (which is possible due to "Allow growing"):
Attachment:
Crop1.jpg [ 47.78 KiB | Viewed 2466 times ]
* The transparency is added around the current image, there is no need to add an alpha channel:
Attachment:
Crop2.jpg [ 49.61 KiB | Viewed 2466 times ]
* Export to some format that supports transparency of course.