The Geologica mod uses overlays to render its bricks and cobble variants, via the default block renderer but with getBlockRenderPass() returning 1, and uses canRenderInPass() to know which icon to return. I think all that is needed is a call to canRenderInPass() here instead of the current check based on ==. I could try it and submit a PR, if you think it's a workable approach.

Seems like it should work. Try it out and let me know your results.