This subject comes up from time to time and I saw this video of one on a JCB. Unfortunately the link to the company is dead and the poster hasn't answered any questions since it was posted in 2010. Just thought it was a cool design idea to share.
NH575E, Same place I purchased my Amulet. Looked at them prior to the Amulet, but figured with the cost of adding the hydraulics, I would go with the amulet instead. https://equipmentland.com/product/ami-hydraulic-thumb-for-extendable-stick-backhoes-ami-exsth/