Add a Flag called "Hidden". When "showing all" by default don't show "Hidden" photos.

In the main toolset, have a new flag called "Hidden" (in the "Picked / no flag/ rejected" section). 

In the settings have 2 toggles: 
- Default "Show All" view shows Hidden" photos: Yes / No 
- Password protect "Hidden" photos: Yes / No --> If yes, record a password which will be promoted to change to option 1 above and also when wanting to show photos marked as "Hidden". 

Very simple, this shouldn't take more than a couple of days to implement and test.
This will add value for hundreds of photographers.