Crap. A perfect answer to my question is in RobA's wonderful tutorial on this site. That's what I get for only doing a quick read on it before making this post. Lolz!
Straight from his tutorial:
Select all (ctrl-A), then reduce the selection by 40 (Selection->Shrink->40). Now
distress the selection (Script-Fu-Selection-Distress). You might have to play a bit to
get it to look OK for your image size: You can also get the selection less rectangular
if you start with a selection made with the lasso tool:
In my version, I had to go to Select->Distort to get the Script-Fu distress thingy.