Hey,
I don't know if this is useful to anyone, but for my current project I wanted to turn some PNGs into REX files. So I threw together a tool, and put it up on GitHub in case anyone can benefit from it. You can find the tool here:
https://github.com/thebracket/png2rexBasically, it lets you turn:
into this:
There's nothing super clever about it. It reads an RGBA PNG, and converts the RGB parts directly. The Alpha channel is transparent at 0, 1/4 filled at 25%, 1/2 filled at 75%, and a filled cell otherwise. Primitive, but it gets the job done!