Dang, Rad...you been hangin' out here since September 16 without a single bump. I didn't even know you were here (not surprising since I hang close to the comfort of the pond building forums).
So, the first thing that comes to my mind is the ejector pumps that are used for downstairs bathrooms. If it were installed inside a cage like a live box, it would act as a pre-filter (you know, the 2 or 3 ft rectangular boxes made of galvanized mesh that get tied to the side of the pier). You might attach a couple PVC pipes as vertical handles to lower it the bottom. I fear, tho, that those motors may not be designed for continuous use; not sure. The impeller design is perfect, capable of moving chunks up to 1.5 - 2", so there would be no possibility of a log-jam.
If you google ejector pump , you're gonna find alot of info, including some killer commercial grade units. Matching killer price, too, I imagine.