Well kudos for those suggestions. I came up with this. A few things I changed or added.

I used the eraser and clone tool a lot on the mountains. I just plain needed spinal ridges in exact locations and couldn't leave it up to a math algorithm. Likewise the continental shape (this is, again, a kitbash of a recent commercial product) is a selection outline.

I used only the ocean layer for reefs, didn't need more than that and simply used the eraser liberally. I've noticed in whole-earth nasa pictures that the amount of reef area visible from distant space is patchy.

The reef layer did double duty. I applied a gradient lighter than the ocean color via image adjustment and erased segments until I got a nice continental plate falloff. It'll look cool for fantastic stuff, but I'm not sure if that's visible from space so I'll remove it in the final image.

I also used several segments in my other desert areas. It took awhile to figure these out but I think I came up with a scheme.

1) I drew a very squiggly selection line, going around mountains and terrain features in ways that could sort of make sense (basin areas for heat collection, etc... who cares what the coriolis forces or albedo are doing, I'm restricting myself to the existing map and, well, it is sci fi).

2) Feathered the selection by 5-10 pixels depending on size of area.

2.5) Already had several copies of the reef layer. Did a couple more difference clouds filters to taste. Mostly just stuck with what I had.

3) Inverted selection and deleted a copy of the reef.

4) Applied a desert gradient. See the tutorial for desert color suggestions. Did use hard light blending. Played with the scale to taste.

4.5) On a black 128x128, I already created a grey crinkly paper template by doing multiple difference clouds and then an emboss filter, exporting by selecting edit > define pattern.

5) Applied the new layer (named Desert 1, 2, 3, etc for each one) via normal blending.

6) Stretched and adjusted the image at its edges by alternately using a soft eraser and smudge to taste, getting the desert into the nooks and crannys of the mountain ranges.

7) Applied the crinkly paper texture as a layer style, using hard light blending and scale to taste. Also applied bevel and emboss layer style: smooth, down, size: 2, soften: 4, highlights and shadows to taste.

If you want a scrub desert or some other feature, like forest, the same techniques might be applied along with the texturizer filter, using the sandstone texture to taste. It makes things a little less realistic, but if the viewer goes "oh that's a forest", it might be worthwhile.