Create a png file with transparency and save it with the name brush_x.png (replace xxx with a number). Copy the png into a folder with the name "Gizmos" below the Assets folder of your project. Restart Unity.
It should, I tried it. There's a free asset in the store. Maybe you can try this and see how (or if) it works. https://www.assetstore.unity3d.com/en/#!/content/4189