IJ> Is the users.dat file universal between platforms... Can I copy the one
 IJ> from my Pi over to my Win64 mystic??

Yes, you can copy all your files between various operating systems.  They are all compatible with each other.  For users.dat you should be able to simply copy it.

If you are copying other things like the mystic.dat or file/msg base configuration (anything that has a path/directory configured within it), then you need to make sure you remember to change all of those to point to the directories on your Windows system after you copy them.

--- Mystic BBS v1.12 A48 2022/02/15 (Windows/64)