I haven’t tried converting those to MSFS yet, this goes for other sims and PBR workflow in software like Substance Designer/Painter & Blender.
I had no idea this wouldn’t work in MSFS. 
Whichever method, you’re absolutely right in just using underlying ground brightness will limit an otherwise awesome snow effect, so being able to have more control over this would give 3p-devs the ability for groundbraking results. Coming from a snowy part of the world, I’d love to see more of this.