I believe I nailed V8 (aka true iray skins)

tested with daz studio 4.12.0.86, blender 2.82, plugin commit 044dc16

First let me tell it was a lot of fun to play with this and I also learned a lot about iray and cycles in the process. Then I have to thank engetudouiti for his posts on sss that also helped me. Well now let's get to it.

The issue with iray skins, and why we couldn't get it, is iray doesn't use sss. Yes you read it right. What is called sss in the uber shader is really a true volumetric solution. This is also why iray is so slow rendering skins. It is also not physically correct since skins are not volumes, but since it's the iray way we have to follow.

So below there's an implementation of the iray sss. It uses translucency as we already do, together with volume absorption and scatter to mimic what iray does. Please note that the uber shader gets transmitted and scattering distances, while cycles gets volume densities. This may seem conflicting at first, but the uber distance is really a distance from the surface where the transmission or scattering effect is at 100%, so it really defines a density and it is possible to convert to cycles.

# if there's transmission then use volume absorption
if transmitted color <> black/white
    use volume absorption
    color = transmitted color
    density = 200 / transmitted distance

# if there's sss then use volume scatter
if chromatic sss color <> black/white
    use volume scatter
    color = invert(sss color)
    density = 100 / scattering distance
    anisotropy = sss direction
# please note that sss amount can also be > 1 in some characters
# this is fine since color slots accept values > 1
if mono sss amount > 0
    use volume scatter
    color = sss amount
    density = 100 / scattering distance
    anisotropy = sss direction

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Please note that for this implementation to work we have to setup a good light path solution in cycles. Because both translucency and true volumes are used. So it is especially important to have good values in the transmission and volume max bounces.

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Please also note that this solution will not work with eevee, since eevee doesn't get true volumetric features. Then I'm not an eevee expert so this is it afaik. For this reason I'd propose to let the old sss solution as an alternative for eevee, or for anyone who prefers to get sss rather than volumes for performance reasons.

If I may I'd also suggest to add a third option for skins, that is, to use only translucency ignoring the iray volume properties. This will de facto work as if thin walled is on. The reason for this third option is that for many characters, especially those converted to iray from G1-G3, it seems using the translucency alone works quite fine. Much better than the actual sss implementation that never matches iray anyway. One example if the G8F.

skin options:
- volumetric (iray)
- translucency only
- sss

Actually I'm working on translucency and sss and I feel I may get something better than the actual implementation. Though it will not match iray but hopefully will be closer. I'll open a new issue for this when I'll have some results.

Finally below there are some examples of skin conversions by using the volumetric implementation. First iray then cycles. I only fixed the torso materials in cycles, so you can see the new volumetric skin vs the old sss one. Please note that though the volumetric skin is always much better, in some cases there's a little difference with iray. I believe this is due mainly to tone mapping and tranlucency, for which I'm working to may be get better factors.

EDIT. G1-G3 characters were converted to the uber shader before exporting. This is because iray anyway does it in the background when you render 3DL shaders. To do this just select all the materials in daz studio and apply the uber shader, it's really a couple clicks.

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G1F and V5

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G2F and V6

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G3F and V7

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G8F and V8

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Some G8 characters: Emiyo, Gia, Kala, Lali, Mei Lin, Penny, Vivian

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