I'm making a multi pass shader in Unity, and I would like certain triangles to skip some passes. I cannot do this through multiple materials, as the ordering of passes would not be the same. For instance, if I have 4 passes ABCD, some triangles will be drawn using ABD, and some using ACD. All "A" passes must be done, then all "B" passes, etc... So it must be done within a single material and a single shader.
What is the most efficient way to do this? I could texture map my object with a black/white image and use it as an alpha value for the pass, but this seems sub-optimal.
The most efficient way to do this is to use submeshes. There are nothing magically special about material passes. You can safely replace them with simple one-pass materials and use of submeshes instead. Unity preserves submeshes order when drawing (there was a bug in 4.2, but it was fixed) so everything should be fine.