I tested out the patch and in most cases it works great. When painting more intricate areas with a bigger brush radius I noticed some issues though.
This can be seen when painting for example the ear of of the monkey object with a big brush radius and a sharp falloff to see more of a gradient:

On the bottom horizontal seam there are artefacts which can then also be seen on the mesh, since they start directly on the seam.

When painting with a texture these can also appear easily:

For this example I used a texture with teh area plane mapping and with rake enabled. But even with tiled mapping these artefacts are likely to appear.

Think the solution should check the winding order of the triangle it projects on. Based on the winding order differences it should switch the order to project to the right location.
Will try to reproduce and see how to detect the winding order.

@Brecht Van Lommel (brecht) for averaging upto 4 pixels my first attempt would be to store the source coordinate as float and use linear interpolation with a mask. This might introduce artifacts when projecting near edges. If this isn't working we might need a way to source the color from pixels that might not be neighbors.

Other thing I am considering (separate patch) to determine the 3d position from the center uv coordinate. Currently it is the bottom left. This this would reduce some smaller artifacts as well.