Well, I had a hunch on this and thought I'd try it. Counterintuitively (unsurprising, since this whole thread seems counterintuitive), I applied my V4 skin face setting to the skullcap, then re-added my transparency settings.

The result: the splotchiness is almost completely gone.

The problem of course is that you can't really distribute any such materials on the skullcap, because you don't know what the user is going to be doing with his character's skin.

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Hardware: AMD Ryzen 9 3900X/MSI MAG570 Tomahawk X570/Zotac Geforce GTX 1650 Super 4GB/32GB OLOy RAM

Software: Windows 10 Professional/Poser Pro 11/Photoshop/Postworkshop 3