Com a ajuda de este tutorial , Agora consegui criar o plugin quase exatamente como gostaria.
#**
# Import the relevant modules
#*
from gimpfu import *
#**
# Preform the main script function
#*
def add_border_to_selection(image, drawable, grow_by, border_thickness, border_feather) :
pdb.gimp_selection_grow(image, grow_by)
pdb.gimp_selection_border(image, border_thickness)
pdb.gimp_bucket_fill(drawable, 0, 0, 100, 15, TRUE, 0, 0)
return
#**
# Register the plugin
#*
register(
"djg-border-and-fill",
"Border and Fill Selection",
"This script adds first grows your selection, then adds a border, and finally fills the border in red.",
"David Gard (DJG-Dev)",
"GPL V2 License",
"October 2014",
"<Image>/Select/Border and Fill...",
"*",
[
(PF_SPINNER, 'grow_by', 'Grow selection by (px)...', 2, (0,5,1)),
(PF_SPINNER, 'border_thickness', 'Border selection by (px)...', 2, (0,5,1)),
(PF_TOGGLE, 'border_feather', 'Feather border edges', TRUE)
],
[],
add_border_to_selection,
)
main()