Can't seem to find my completed image in Lighttable.

[kofa (István Kovács)]

Maybe you could write a script to do that, taking this as a base (simply skip invoking Gimp):

Adding images to library automatically upon export

There is a lua script, gimp.lua that lets you export an image and open it in GIMP. When you finish the edit, simply overwrite the file, then close GIMP. The image will be moved into the original image’s folder and grouped with the original image.