i can now overlay a block breaking decal on blocks, and it works on blocks with shell texturing and even nominally on objects like ferns and flowers
i draw the block's faces again using the stencil buffer, then draw the breaking decal with depth read/write off, masking using the stencil buffer
the way it works for ferns/flowers is i just draw all faces of the breaking pattern block, and any pixels outside the object model are masked
a fern with some areas darkened, though the pixels of the breaking pattern don't match up with the fern's texture's pixels
leaf block with block breaking pattern overlaid. it can be seen despite the pyramidal leaf shell texture that sits in front of the actual block face
a tree log block with a block breaking pattern overlaid
a dirt block with a block breaking pattern overlaid