Have seen people use hay bales to grow in, but you do have to condition them first. I guess it depends on your situation if that has to be a purchased material or not, but I am curious if the same idea can be used with other materials. I think the biggest "issue" with uncomposted organic materials is the nutrients are not as readily available, but I've heard that some things like squash love some uncomposted organic material to sink their roots into. I bet you can have success there, just maybe not the most optimal yields or anything like that.