Non Square ASTC Block Size

It looks like Unity only supports square ASTC block sizes from here - https://docs.unity3d.com/ScriptReference/TextureFormat.html

I need to have a bit more granularity in the ASTC compression quality and found that 8x6 is the perfect compromise, but it appears as though Unity doesn't support it currently. Are there any plans to support the non-square block sizes?

More info on the various block size / bits per pixel here - https://developer.nvidia.com/astc-texture-compression-for-game-assets#_Toc398571911

Thanks, Dustin