It'll cut it I'm afraid, just copy the layers that give the effect you want and undo to go back to before you script-fu'd and paste that into your sig. You might also want to increase the size of the text layer (layer -> layer boundary size) or it'll cut off some of the glow too...
Also contour select is the little wand on the top row of the main gimp window to the left of the colour selection. (Its actually called contiguous regions, sorry if that confused you).