I feel dumb, just took some more playing around to realize that Photoshop's pixelate -> crystalize filter is great for this.

Make walls black on white, crystalize with a high number, crystalize again with a lower number to break it up further, adjustments -> threshold to reduce again to black and white and you have a jagged mask you can use on anything.

Click image for larger version. 

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Also I didn't notice you were an admin before! Thanks for taking the time to reply to me even during yesterday's problems with the site.