There is no direct way to do this, it's an interesting question though..
The fastest way to currently do what you want would be:
1. Select region, choose crop, then Save+ to save the cropped image as new screenhot with autoname, then copy image to clipboard.
2. Select region, choose "Save selected region to new file", then select that new file and copy to clipboard.
It might make sense for me to add a new menu item in Edit menu copy to clipboard that said "Copy selected region to clipboard as bitmap"
But that wouldn't exactly answer your question if you need the region copied to clipboard as a FILE.