This doesn't seem to work, as the center of the pasted screenshot is centered in the canvas.
This also reminds me of a similar use case in which I paste and save the screenshots, then go back later to crop.
I think one way I've done it before is to paste all the screenshots in as new layers, then created the selection and cropped, saved as .pdn, then went through and selected each layer one by one and saved it to a file. I guess thats probably the best way to do it.