Technically using an encrypted .mtl would already work, it's only the UI tool that doesn't know the .mtl file-extension yet.
Also the command line tool doesnt like it:
D:\temp\panotest\3DGallery_06\models>D:\dev\krpano\1.21\krpanotools encrypt gallery_06.mtl
Error: gallery_06.mtl => gallery_06.encrypted.mtl - unsupported file type!
So what I would have to do is rename it to a supported file type, encrypt, and then rename to the original .mtl ?