Undo

An interesting script but have you considered adding an undo feature to it so that the effects of changes to the control can be evaluated more easily (by just pressing Ctrl-Z rather than having to delete the 2 layers the script creates)?

Changing the start of the code to read:

(let* ((layer-new 0)(layer-new2 0)(layer-new3 0)(mask-new 0)(curve-new 0))
(gimp-image-undo-group-start image)

(set! layer-new2 (car (gimp-layer-copy drawable 1)))

and the end of the code to read:

(gimp-displays-flush)
(gimp-image-undo-group-end image)

does this.