Dad has an old hammermill feed grinder we've converted to chipping brush. I'm not as thrilled with it as a brush chipper as he is, but it works okay (definitely not OSHA approved and to be honest it sort of scares the daylights out of me). But, it would work great for doing what you propose.
Start looking around at farm sales and you might come across one for basically scrap price. I'm almost certain you'd need to beef up the screen and likely make some modifications to the feed mechanism. But, it wouldn't take much and you'd be in business, I bet.