Replied in the other thread, but long story short is that support for multiple textures per sprite is planned for 4.8. It won’t be specifically for Android texture compression, but with a custom material that pulls RGB from first slot and A from second, it could certainly be used for that (as well as for normal-mapped sprites, complex materials that need a mask texture, etc…).
Note: You can work around now if your source assets are already atlased, you could create one material instance per atlas page that references separate alpha texture as a non-sprite texture sample. (You could even do it if they aren’t atlased, but you’d have a plethora of loose one-off materials that might be annoying to work with).