I would hardly recommend, that distributing textures randomly on a Mesh, is a task you would perform in Unity.
Theoretically it is possible to distribute textures randomly on a Material. But if you don't bake them to a simple UV-layout, I am afraid, you wont be able to ship this material to Unity.
But I would ask this question on a Unity focused platform.
But if you want to do something like this in Blender, I think this is only possible with the help of Python scripting or an osl shader.
Even if this Question will be closed, I have to share the result of my tests, because if not, I would have wastet to much of my time.
The closest result wich I found was to procect multible textures onto the mesh like decals. Therefore you have to use external objects as texture Coordinates.
The results can only be seen in the render mode.