vertexAlphas without vertexColors?

r127 enabled us to encode per-vertex alpha into the color bufferattribute. I’m looking to build a mesh of varying opacities, but all of the same color. Is there a way to save 75% of the overhead by cutting rgb out of the buffer (perhaps sticking it back into material.color) and keeping only the alpha channel?

I can write a custom shader to do this, by bringing together an attribute float alpha with a uniform vec3 color, but was wondering if I could finagle this with built-in materials.

No, this is not possible. You need a custom shader for this.