Covered in stones

I posted this in the "Editor & General Support" section, but this is probably a better place for it.

I need to distribute hundreds of thousands of stones onto a riverbed object.

• I was hoping I could do it with a particle system (like I can do in Max), but I guess that isn't possible in Unity.
• Unity's Terrain tree-painting tool almost does what I want when I load my custom stone objects into it, but it doesn't seem to do random scaling and rotation, and that's really essential.
• There are some other third-party scattering tools that technically do what I need, but every single object placed gets its own list entry, so for me that won't work. as I need to place so many.
• A normal-mapped texture is fine for a base, but this client is especially picky about realism, so I really need to build things out in 3D for this.

I have some ideas of things to try, but hopefully there are some faster, simpler solutions possible. Unity isn't my main software. It's just something jobs require from me a couple times a year.

Thanks for any advice you can give.

 

When I designed my Mars level, I used Blender's particle system to distribute the rocks randomly then I re-exported the model in fbx with the rocks in the ground. I just then applied the respective texture of each rock.