Yea, I thought you might already know, but I wasn't sure. (And I don't use GIMP, so...yeah >_>). Hopefully I didn't come off as demeaning or anything; I wasn't trying to do anything like that.
I know that in Photoshop you are able to create a custom brush that actually paints on a border. Basically, instead of it acting like a normal brush, it acts more like a huge border stamp so that it paints on the border with one click. It works like this
in Photoshop. Maybe it would be possible to do something like that in the GIMP? Like I said, I don't use GIMP so I don't know; I'm just throwing thoughts around.
... yea ...