I think I cracked it. It seems that the object is one of the small doors covering a fuse box. The size of the panel and the height of the handle seems to match with the design.
I don't know much about the electricity circuits aboard tho, so I cannot say whether or a not this placement for a fuse box would actually make sense or would the box even fit inside the wall.
This would confirm that the entrance was at the forward end of the vestibule but I still wouldn't rule out the possibility of a door or a some sort of cover existing instead of a simple opening since there's still some evidence supporting it, such as the dark corner, Browne's drawing and the fact that this was a private bathroom so the opening doesn't really make sense. Are these enough? I'm not sure.