Mappin' mappin' mappin'. In Inkscape, because that's the art program I know best by a good margin.

The Pixel Smear filter gives a good coastline-y edge, but slows down processing tremendously.

So, think I, I can export this and import the image as the base layer of a new file, where I can add rivers and climatic zones and signs of civilization and so on.

Which works, but. Importing is limited to the resolution at which I exported, meaning the coastline is all pixeled edges once I zoom in.

Also I have no idea how well the kind of double-export that will result from this will go.