I'm struggling a bit with the challenge of getting some shading, in this case specular, over a 100% transparent surface in the context of a non-photoreal shader. I know Mantra doesn't conform to a ‘priority fill’ type renderer but I would still like to try to get it to work.
I would like to avoid rendering twice for the shading I need over the transparent objects.
As long as you're using “raytrace” render engine with stochastic transparency disabled, it should be doable. (in your material, feed 1 into “Of” input of Compute Lighting VOP, and 0 into “Of” of surface output.