Eric,
I am a new backhoe owner and have been thinking the same (about a
thumb) for the hoe. Looking in the Machinery Trader and online, there
are fixed and movable thumbs. Hunt around your area for someone in the
demolition business. They would be a good source of info from someone
who uses one every day. Might even let you try one on their machine.

One interesting movable thumb utilized the extenda-hoe movement.
Others required aux hydraulics.

Bob