«Text border» plugin for GIMP (put it to `~/.config/GIMP/{version}/scripts/`

text-border.scm

(script-fu-register "script-fu-text-border"
 "Text border..."
 "Creates text border" 
 "Anon"
 "Anon"
 "2012"
 ""
 SF-IMAGE "Image" 0
 SF-DRAWABLE "Layer" 0
 SF-COLOR "Color" '(0 0 0)
 SF-ADJUSTMENT "Width" '(5 1 45 2 5 1 0)
 SF-TOGGLE "Auto width" TRUE
 SF-VALUE "Auto width divider" "13"
)

(script-fu-menu-register "script-fu-text-border"
                         "<Image>/Filters")
                         
(define (script-fu-text-border image layer color width auto-width auto-width-divider)
  (let* (
        (foreground-color (car (gimp-context-get-foreground)))
        (image-height (car (gimp-image-height image)))
        (image-width (car (gimp-image-width image)))
        (text-mode? (car (gimp-drawable-is-text-layer layer)))
        (original-name (car (gimp-drawable-get-name layer)))
        (border-layer)        
        )

    (cond 
      ((= text-mode? FALSE)
        (gimp-message "Error: Active layer is not a text.")
      )
      (else
        (gimp-undo-push-group-start image)
        (if (= auto-width TRUE)
          (set! width (/ (car (gimp-text-layer-get-font-size layer))
                         auto-width-divider)))     
        (set! border-layer (car (gimp-layer-new image image-width image-height 1 "border" 100 0)))
        (gimp-image-add-layer image border-layer -1)
        (gimp-drawable-fill border-layer TRANSPARENT-FILL)
        (gimp-selection-layer-alpha layer)
        (gimp-selection-grow image width)
        (gimp-context-set-foreground color)
        (gimp-edit-fill border-layer FOREGROUND-FILL)
        (gimp-context-set-foreground foreground-color)
        (gimp-image-lower-layer image border-layer)
        (set! layer (car (gimp-image-merge-down image layer EXPAND-AS-NECESSARY)))
        (plug-in-autocrop-layer 1 image layer)
        (gimp-drawable-set-name layer original-name)        
        (gimp-selection-none image)
        (gimp-undo-push-group-end image)
        (gimp-displays-flush)))
      )
    )