I had noticed a certain lowering of the tone whenever he pops up.... though the firefly references add a whole new depth.

I'd sketch the shape you are after roughly on one layer, not worrying about much other than a nice shape. Then I'd locate major features like rivers and mountain ranges on a new layer. As Ghalev says, note how they affect the coastline - google maps is the best way to nail this. Then rebuild your sketch on a new layer with these effects in mind. That should give you a believable shape. Finally. go around the coast carefully with a fine brush and allow your hand to squiggle as you do. I find that my own hand jitter is lot better than any built in jitter.

I don't have art rage, so these are my steps in Gimp. They should work fine in other programs though. It's also an interesting exercise to import a real image of a coastline and trace around the edge. You can always rotate and/or flip it, to make sure no one will notice where it is.