Unity Prefab Placement Editor is an extension for Unity 3d that allow you to paint any prefabs(most useful for vegetation) in any mesh that has a collider component. It has similar options than the built-in terrain tree and grass painter.
Import everything to your Assets folder, PrefabPlacementEditor must be inside Editor folder. Create an empty game object and put the component PlacementSystem script on it.
All prefabs that you want to paint need to have separated Tag in order to make the tool work properly. You can use the tag to separete trees, grass, rocks, bushes and so on. That's the way I found to avoid ambiguities when using the erase option.
Select the game object with PlacementSystem to enter in paint mode. A blue circle will follow you mouse through the ground surface(the ground need to have a collider). To paint hold the left mouse button and drag them over the area, to erase, do the same while holding left shift button. The undo and redo shortcuts are default.
Prefab is a array of game objects that you want to paint, you can paint multiples at once.
Layer Mask is the layer of the surface that you want to fill, no needed to create one, just let as default.
Prefab tag is the tag of the objects that you want to erase(hold left shift).
Randomize prefab let you paint multiples objects in certain array range. If it's off, you can choose by array index.
Override let you put one prefab over another, doesn't works very well. Turn it off.
Spread, radius and amount are the fill options.
Aling with normal let your prefab follow the ground rotation. It's useful if you gonna paint in planet surface.
yOffset is useful to avoid misplacing, very rare glitch. 2 is a fine value.
Randomize size and rotation, options to avoid bad tiling. Useful when you gonna paint some trees.
Hide in Hierarchy hide after instantiate. It's not necessary at all, prefabs are automatically child of the surface.
TL;DR: Left mouse button to paint, shift + left mouse button to erase.