I seem to have gotten it to work in a different way. Basically all I have to do to target a defined BMAP and apply a view shader is :

bmap_process ( sprite_A, sprite_A, mtl_emboss );

It applies the material effect to the BMAP. Changes apply to any panels using the BMAP.

Q. What material code do I use to make the sprites transparent?
When applied to a window sprite, it colors the transparent pixels.

(Note : Not all view material work since they don't seem to be set up for this purpose. Only certain materials like "Negative" seem to work, but transparency is not correct.)