filltbl isn't a macro, nor does it affect the global namespace. As written, it just creates a table if its first arg is nil. Of course, this is useless unless it returns that table... oops.
You could push it onto Anarki. Other than that, there isn't much you can do to get it into a public arc.arc. It seems that if a function isn't used in the forum code, pg doesn't include it in the official release.