The Ruby API only lets you set a layer/tag's color.
[Note in v2020 'layers' were renamed 'tags', but in the API they're always called 'layers'].
But you can't [yet] set a layer/tag's material [i.e. there's no access to textures or to transparency settings].

However, in Ruby you could set up a separate color for the transparent layer [but setting a alpha value does nothing] and when it's set you do a simple edit for the layer/tag's color in the Layer/Tag Browser, and NOW you can add some transparency...

model=Sketchup.active_model model.start_operation("LayerColors",true) model.layers.each{|layer| if layer.name=="0_EX_Glazing" #grey to get transparency setting later on... layer.color=Sketchup;;Color.new(150,150,150,50) else # 0128_White ? Set up any RGB you want ! layer.color=Sketchup;;Color.new(255,255,255) end } model.commit_operation

Ensure your Model is set up to use color by layer/tag - in the layer/tag-browser pop-out...

Run this in the Ruby Console...

Next manually edit the "0_EX_Glazing" layer/tag color in the layer/tag-browser to have some transparency...

There's no option for 'back' material in layer's colors...

I think you're wanting to make SketchUp behave like Revit/AutoCAD but it works differently.
SketchUp needs to group geometry to keep them apart. Without groups/components you would end up with a model that is impossible to work with since edges and faces would be glued together into a gigantic messy mesh.

I have been working with SketchUp since 2000 and in the past I also tried to put loose geometry to different layers but since they are connected there WILL be problems.
Having all geometry on Layer0 and putting groups and components to different layers is what works best in SketchUp.

As a side note, I have found that SketchUp somehow is much faster if you have grouped geometry versus loose geometry.

Here is my current best workflow Revit to SketchUp:
https://sketchucation.com/forums/viewtopic.php?f=15&t=68000&hilit=+Revit