Latch or Sash Pin
These were often used to lock doors and windows from the inside similar to a night latch in function.
Want to keep that tradtional look and not have thumbturn, push button or key cylinder or use with your thumblatch? Try a latch lock pin.
These will keep you bathroom door thumblatch secure by installing above latch bar. The latch bar can no longer be raised and therefore the door is locked