We're adding M4B support for file display columns in the next update.
I'm hesitant to add it as an alias for the metadata panel (i.e. as read-write, not just read-only) because I think there are some differences between M4A and M4B files, or at least their tags, and I don't know enough about them to know if doing that would be safe.
(In particular, I don't know if the tag library we use would wipe your bookmarks or current position from the file if it modified the tags. Reading them is safe, but I'm not sure about modifying them.)