Seems like you could create a custom content type based off of a list and
specify that. I haven't done it yet though, and will be watching to see what
others say.

The content type approach was my first thought as well.

I don't have an answer for you on this one but I wanted to point out that you
could have a category that is set by default on the list. Make a required choice
field with a single entry and make it the default setting. That way the users
don't have to set it or change it. You could even hide it using management of
content types or JavaScript on the form.

Or have the "category" simply be a list meta-data column (as you did not want
to do), but required, and populated by default to be the unique "category" of
each such list.