MLLE 2.21 is out! It adds support for 32-bit tilesets.

What's not currently supported is editing individual 32-bit tiles inside MLLE. Unlike 8-bit images, there is no common standard for copying 32-bit images via the clipboard. It would help tremendously if people could report (in this thread) which image editing software they use and, ideally, what clipboard format it uses. You can use Free Clipboard Viewer. If your software doesn't support MLLE's existing copy/paste support for 8-bit images, I'm interested in knowing about that too.