Update on the above. Turns out these are made only about 10 miles from where I live by a guy I met this spring, Scott Lord. He demo'd one for me this morning on a L2500 Kubota with an LA452 (?) loader with 60" utility bucket. Even got to use it for a few minutes myself. Very nice and practical little unit. If I understood correctly, the demo unit was the prototype first made in 1999 and has seen alot of use (and, according to Scott, abuse). He's made a number of these, some with two cylinders for use on quick-detach buckets.
It worked fine and looks like a good match for any Kubota utility bucket. He makes them up custom using measurements from the customer's bucket because Kubota bucket dimensions vary from year to year. He has also started making them for NH buckets.
The 'claw' portion is easily detachable from the bucket and looks to weigh a little less than 100lbs. The cylinder and claw attach fittings stay attached to the bucket.
Haven't seen any numbers on the price yet, but expect them to be competitive. He provides the claw with cylinder, valve, and valve-to-cylinder hoses/fittings. Valve to tractor hoses have to be provided by customer as I recall.
Scott seemed to be able to pick up almost anything he wanted with the claw and control it easily. He demo'd the unit on a pile of broken up skids. It wasn't quite so easy for me, but then I haven't used a front end loader for quite awhile and needed to acquire the feel. I also thought the unit would benefit from having a toothbar on the bottom of the bucket as it would allow you to get under the object being picked.
Expect to be learning more on this shortly. Will update this thread as that happens.