A simple solution may be the “bathroom availability” lights that I’ve seen on some planes. If the bathroom is occupied and everyone in the main cabin can see it

A simple solution may be the “bathroom availability” lights that I’ve seen on some planes. If the bathroom is occupied and everyone in the main cabin can see it, they’re less-likely to get up to go to the bathroom, especially considering the aisle is only wide enough for one person. Of course the pilot should be able to flip the status to “occupied” before they even get up, that way no one gets up as the pilot is making their way out, and it kind of gives them priority (which they should have anyway for obvious reasons).