This September update added an example prefabs and scripts that use painter to... well, paint stuff. A dirty ball, bullets, guts and stuff.
In the picture at the top, left object is using Big Render Texture pair, right one - exclusive RenderTexture. You can paint on many objects with Worldspace brushes, but they need to have Render Textures.
To be exact, new version of Sphere Brush actually renders a capsule, not a sphere. So if you move a brush or this painter ball very fast, it will not render circles, but a line between where it was last frame and where it is this frame. It uses formula to find the closest distance from a point to a line, nothing too difficult.
Now you may be thinking "Can I use it to render damage onto a character, like from a bullet." Answer is yes, you can (video below). This is an early stage, shader will be available with the next update.
Actually, it doesn't paint dirt and blood onto a texture, it just paints green and red color to a mask: green - dirt, red - blood. And shader uses that mask to cobine character texture with gore and dirt texture. Mask is also sampled in vertex function to indent mesh. Ideally there would be another mask with normals and max indent amount to really control deformation.
In terms of performance: painting a sphere brush to an Exclusive RenderTexture will take same time as rendering a quad to a screen that has resolution of that Render Texture. It takes much more for RenderTexture Pair though. For regular brush, not sphere, it's much much faster, just like rendering a quad . Swapping render targets could eat up some performance though. But in general, performance is fine.
Painter ball currently modifies only one texture at a time, to avoid additional renders. But you can make your own painting scripts so be mindful that when you try to paint on texture2D using RenderTexture target, it will first render texture2D to RenderTexture, and will have to do this again if you change paint target on snother texture.
What is Render Texture Pair
When using Render Texture as target, you can't read from it, but RenderTexture pair contains two copies of the same texture, which is updated after every stroke, so almost all possible brush effects can be reproduced with shader because fragment function can sample destination color (from copy).
Thankfully, Sphere Brush Type doesn't need the pair, so it can paint on single(exclusive) Render Texture as well as on the Pair. Only Blit modes are limited: no Blur, Bloom or Volumetric Decals when using exclusive Render Texture.
Tanks to Daniel for embracing Painter Tool and providing me with valuable feedback. He is using RenderTexture Pair here, I think.