It would have to be ‘nearly flat’, with maybe a % no or to define ‘how flat’ - from 0% = truly flat, up to say 15% = some definition of waviness. RC might call it a ‘weighting’, as elsewhere, so RC knows how rigidly to act upon the ‘this is flat’ instruction. Because, in buildings at least, surfaces are rarely truly flat in reality.
The ‘by single click’ would be a challenge to impliment; drawing polygons would be a huge task, multiple flat surfaces on multiple photos. I guess it would have to be done within RC after loading, and would not be possible to transmit and save the markings back to stroage in Windows Explorer folders?
What about perhaps multiple complex objects, like wall-light fitting, on an otherwise flat featureless expanse?