That would be a great scene!
Chekov: Permission to leave the bridge, Captain?
Kirk: What for?
Chekov: I have to take a dump, Sir.
I can't decide if that's better or worse then the TNG alternative, with the head right there
. Right in front of the rest of the bridge crew. No wonder Picard has his own in his ready room.
I always liked the idea of there being one in that gangway around the TOS bridge though. It's a clever idea.