Okay, a further update. First, I should let you know that this is a product that works (displays correctly in the Preview mode of the Pose Room) on my computer, but has exhibited this bizarre behaviour on a reasonably fast system that has 4 gig of RAM, updated OpenGL drivers, the latest SP for Poser 7 installed, and a savvy Poser and OS user sitting at the keyboard.

What is puzzling is this: the textures appear on the preview pane of PoserSurface! The textureMap of the main colour jpg is attached to the Diffuse_Color channel (that was connected all along), with Diffuse_Value set to 0.
Adding an Add(0,0) node and attaching it to Diffuse_Value (set to 1) does not change the preview in the Pose Room in the slightest: it stays white.

Textures from other developers with less "elaborate" shaders load and display on the figure correctly.

Monterey/Mint21.x/Win10 - Blender3.x - PP11.3(cm) - Musescore3.6.2

Wir sind gewohnt, daß die Menschen verhöhnen was sie nicht verstehen
[it is clear that humans have contempt for that which they do not understand] 

Metaphor of Chooks