What is the "book properties window"? Is it what calibre refers to as "Book details"? If not, what?
If you mean Book details then you can do what you want with a column built from other columns that shows the number of extra files (what you called "comment files") associated with a book. Create the column giving it whatever lookup name and column title you want. The column creation dialog will look something like this:
This is the template I used when creating the column:
Code:
program:
count = has_extra_files();
if count then count else 0 fi;
Counting the extra files is an expensive operation. For performance reasons I suggest that you "hide" the column in the book list.