Yes, SF-TOGGLE is what you want for this sort of flag.
So in your register block you would put something like:
SF-TOGGLE _"Do other stuff" TRUE
The procedure define line would be
(define (script-fu-threshold-boarder image drawable threshold do_other_stuff)
Then in your code you could put
(if (= do_other_stuff TRUE)
(begin
; code to do if selected
;Border effect
(plug-in-randomize-hurl RUN-NONINTERACTIVE image boarderLayer 1 1 1 FALSE)
(plug-in-mblur RUN-NONINTERACTIVE image boarderLayer LINEAR 20 0 x-center y-center)
)
(begin ; This is the optional 'else' case i.e. code to do if not selected
)
)
Kevin