Since the 22nd it's either been cloudy or I've been too busy to shoot, so I've spent a bit of time on trying to eek out what I can from my Mars capture from the 22nd. I had an epiphany today: my blue channel is way too compressed with daylight white balance—indeed, it covers less than half the levels of the red spectrum.
This makes it the primary source of noise on the disk. I decided to try aligning the RGB histogram peaks before going headlong into sharpening to perhaps reduce this. It worked for the good contrast areas on the south pole, but not so much on the disk:
A look at the blue channel further illustrates the trouble with trying to expand the bit resolution by 100%:
I think I might try using auto white balance this weekend—this should spread the histogram evenly across all the colors.
P.S. I got a better result on eliminating the edge rind in PS5 by removing it by channel (in a separate document, and then pasting the fix back into the RGB). I created a curved path along the center of the rind and did a simple path stroke on a new lighten layer (15 pixels 0% hardness, 15% flow); I then used layer opacity to fix the darkest spot and then used a 50 pixel brush on a layer mask to feather the lighter areas.