Try looking into the PythonFu Slice filter. It will take an image and then divide it into however many pieces you want in as a even a way as possible, then you can manually re-join them in whatever way you want.
There's also the Guillotine plug-in that divides up the image based on any guides on the image. Again, this would require you to manually composite the images back together.
And, finally, there's this Split and join images filter I found on the GIMP Registry. I haven't got it to work, but perhaps you can. It also seems to be partially in another language.
These aren't exactly what you want, as it's not automatic - but perhaps you can take these and create your own PythonFu script or GIMP extension!