functionality to join images

[blun]

Functionality to join several JPEG images together (like jpegjoin) would be very nice indeed.
Functionality to (automatically?) integrate some (heavily) altered blocks from an edited JPEG file into an existing original file would be even nicer; e.g. for removing watermarks or other edits that only affect parts of the image. (Well, an export option in The GIMP would be even more useful...)
This feature would require a special jpegtran patch ("droppatch"), though.