Stumbled onto this hack. I think Blender really needs an edge mode for texture painting. Select a set of edges and automatically apply a paint stroke to them in the texture painter. But until someone makes that you can use this method.
Step 1
UV map your object. Add a material with a blank texture.
Step 2
Duplicate (SHIFT D) your object so you have an exact copy with the same UV’s, texture and material.
Step 3
Select the edges you’d like to paint on the duplicate object and bevel them. This will create new faces.
Step 4
With the new faces still selected, go into Texture Paint mode and hit the polygon Face Mask button. Now just use the fill tool.
Result
Since the original object shares the same texture and UV’s the edges will be painted, except there will be no extra geometry. As you can see, it doesn’t work 100%, some of the uv’s will need to be adjusted. But it is a pretty quick way to paint based on the geometry of your model.
