Texture brush mapping mode: 3D stencil

It would work in a similar way as the stencil brush mapping mode works, but now the stencil would be a transparent 3D plane that can be scaled and rotated etc in the viewport just like other 3d-objects. So you could locate your stencil in a place where you wanna paint and then "project" the paint to your object by painting through the 3D stencil plane. Maybe there would also be a transparency option with the other settings for this.

Here is a mockup of how the plane would look like. Now you only would need to paint through it, or locate or scale it better.