To clarify, place your free palm on uke's thumb and apply the same extension you would for a regular sankyo. Extending energy and turning your hips toward uke. If you're particularly strong try "crushing" the thumb into uke's clenched fingers. Your idea of bending the thumb into itself to cause pain is pretty close to the idea. Debatable as to whether it's "aiki" but whatever works. I apologize if my exlpanation lacks..., it's one of those "gotta do it to figure it out" things. Good luck!