Hi Mike,
This is a very good question! You are right to say that beans_sub_menu_wrap and beans_sub_menu don't have subfilters so you can target it it directly.
It is added to Beans 1.3.1 which will be released tomorrow so one more day to wait unless you grab the latest version from github 🙂
Thanks,
I've now encountered this, I only want to add some class attributes to the first sub menu. But using beans_sub_menu[_navbar] adds the class to both submenus. Any suggestions?