That’s one way - another would be to draw the polygon in the model within RC.
However I realise that would require a higher level of 3D-modelling type tools within RC. Present rectangle and lasso tools are too imprecise, un-editable, and are vague in the view-z direction.
But still, would be great to be able to do it not to multiple photos but just once in the model within RC. Maybe such a special/limited-purpose 3D modelling tool wouldn’t be too difficult.