Is there a way to make QuickLook display a hex dump preview of files for which no other preview is defined?
I think you would need two things in order to make this work.
 1.  You would need a hex editor that was tied into whatever black magic Apple is using to provide QuickLook.
 2.  You would probably need to set hex editor as the default application for handling the specific file type...   

In short... no, I don't think you can do it out of the box.