I've run some dirt and occasional rocks through my pto powered hammermill, aka bearcat chipper/shredder. As mentioned the dirt and rocks are hard on chipper blades and although not good for hammers it's not the end of the world (unless you are getting lots of large rocks). As long as the rocks are smaller than the screen they will pass quickly. That will handle material up to about 2 inches as long as its not real long. Those shredders work great if you have a lot of small branches, mine will rip them right out of your hand.
For pure reduction using a leaf blower will make a burn pile go quick, much like an air curtain burner used in large scale biomass reduction.
If you are into utilizing waste this might be an option worth considering. Backyard_Biochar