to add custom attributes you’d have to write a custom shader.
the “default” techique for vertex “mask” data is using the vertex color. you basicly have 4 “floats” available for whatever you wanna do. in the source it is specified as 8-bit precision srgb. that’s a lil counter intuitive for non-color blending operations, but you could convert it to linear space in the material shader. and ofc you can crunch more data into it if you use bitpacking or shifty shader math.