Well I did my best.
Your best with limited information, perhaps…
It’s already done
...and so you’re tied in to the broken functionality. I sympathize with that, but don’t try to justify the shader as somehow making more sense this way than it would if it were actually able to be used with all relevant textures [as a shader should].
I’m not really sure how far out of my way I should go to make sure files created for Poser work with a product made for DAZ Studio,
Because you’re advertising this as not just a standalone, but an addon to existing products, and because a tremendous number of textures use Poser-style bump mapping. Because this is something which could, and should, be universal. And because the change required would be relatively minor unless you threw away your shader code after compiling.
and as far as I know Poser is the only program to ever use black as base. I’ve not personally run into any displacement or bump maps made that way in a long time,
Every single one of the vein/brow displacement maps I can think of offhand uses black as the zero color. And they’re the sort of thing which needs to be able to be mixed with bump maps from more than one character.
any that are can be leveled quickly (set black level to 50% gray and you’re done).
In an external image editor. There should be no need for fiddling and duplicate files when “Min=0” could easily have been set from within the shader.