shape key / blendshape vertex normals support

I’d like to help push for Blender to support per-shape key (blendshape) vertex normals, and packing them into FBX and any other relevant formats that support it. I’m working on a project that requires custom normals be imported to unity for each blendshape (shape key). I am aware of some case-by-case alternative solutions (custom shaders, mostly), but i am under limitations that make these non-viable.

I’ve done some poking around forums, and it looks like I’m not the only one looking for this feature.

Essentially, the only options right now for the blender > FBX > Unity workflow are:

1: Import custom normals, calculate blendshape normals. Result: blendshapes mutiliate the vertex normals, especially when more than 1 blendshape is active. this is only useful if you plan on using 1 blendshape at a time, and want the custom normals to all disappear when activating any blendshape.

2: Calculate all normals for all shapes, discard custom normals. Result: functional, but at the cost of all custom normals.

3: Legacy blendshape mode, which uses your basis custom normals on all blendshapes. Result: most functional for most purposes, but blendshapes look unusual with the basis normals applied. many unity applications work best with this if you do not have custom blendshape normals.

4: Import normals, and import blendshape normals. This is desirable, but blender cannot bake these normals into the FBX, and it falls back on option 1 when it can’t find custom normals data for the shapes.