Definitely looks great. Did you start with a feathered white blob as you did with the 'not so random coastline' tutorial?
Would it be possible to put this coastline and a somewhat smoother coastline on the same map without too much difficulty?
D'oh! Why didn't I think of that?

(you don't quite get the same effect with a blurred shape, but...)

Bottom Layer (same as previous example):
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Middle Layer (Screen Blending Mode):
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Top layer 50 % grey (Lighten Blending mode):
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After selecting the 50% grey with 0 tolerance using the colour select tool, and copying selection to a channel:

Variable randomness coast, based on a blurred shape:
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Might have to video this

-Rob A>