Yep! Its really challenging, but its very possible with some clever tricks like mirroring UV's for things to save texture space and using vertex colors for other things that don't need textures.
This is what my atlas looks like for my submission, which i could have saved a lot more atlas space with some cutting of textures in half and flipping them on the UV map. (particle systems were rough too since I had to put the smoke tex into the atlas for it to count)