Oops, the example I posted was for nice crisp masks, but the same principle applies. Here's an example for Specular, Bump and Displacement.

The key is creating the image that combines those maps. I create these as usual (in grayscale), then when they're all done combine them in Photoshop by copying each grayscale map and pasting it into the relative (Red, Green or Blue) channel in Photoshop. It can create a weird-looking color image, but that's not important. If you're using GC, the color image should be linearized (Gamma =  1).

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