Performing multiple operations on image in memory with libjpeg

Question

I have an an application where I would like to perform lossless swapping of tiles in a JPEG, resulting in a file such as this: To do so, I have been able to make a proof-of-concept python script that repeatedly calls the patched jpegtran with crop and drop support like so:

# Export tiles
for i, swap in enumerate(swaps):
    if i > 0:
        continue
    commanda = [
        jtp,
        "-crop", "{}x{}+{}+{}".format(dividerWidth, dividerHeight, swap["sx"], swap["sy"]),
        "-outfile", "tmp/s{}s.jpg".format(i),
        "original.jpg"
    ]
    commandb = [
        jtp,
        "-crop", "{}x{}+{}+{}".format(dividerWidth, dividerHeight, swap["dx"], swap["dy"]),
        "-outfile", "tmp/s{}d.jpg".format(i),
        "original.jpg"
    ]
    print(list2cmdline(commanda))
    call(commanda)
    print(list2cmdline(commandb))
    call(commandb)


# Then drop them on the original
for i, swap in enumerate(swaps):
    command = [jtp]
    if i == len(swaps) - 1:
        command.append("-optimize")

    source = "mixed.jpg"
    if i == 0:
        source = "original.jpg"

    commanda = command.copy()
    commanda.extend([
        "-drop", "+{}+{}".format(swap["dx"], swap["dy"]),
        "tmp/s{}s.jpg".format(i),
        "-outfile", "mixed.jpg",
        source
    ])

    commandb = command.copy()
    commandb.extend([
        "-drop", "+{}+{}".format(swap["sx"], swap["sy"]),
        "tmp/s{}d.jpg".format(i),
        "-outfile", "mixed.jpg",
        "mixed.jpg"
    ])
    print(list2cmdline(commanda))
    call(commanda)
    print(list2cmdline(commandb))
    call(commandb)

This is, however, very inefficient and slow. I started trying to make a version inspired by the source code of jpegtrans (patched vers.) to no avail. I do not understand how I take the cropped image (in a jpeg_compress_struct) after executing (jtransform_execute_transformation) and then dropping it on the image. I seems like I could do this with just one copy of the entire image in memory, and crop out parts and drop them in other places, but I have no idea how to go about this.