Hand tracking in VR is mature enough for such research. You can transplant a finger from one hand onto another by copying over the skeleton poses. Physical colliders synchronized to skeleton position allow you to interact with objects of different shapes. There is no haptic feedback, which is also true for this artificial thumb.
As for real life: while it would be amazing to have a second thumb semi-permanently, I would prefer it to be pure mechanical contraption without any running software.
Mechanical contraption is exactly how I would describe Ian Davis's projects. You should check out his mechanical hand if you haven't already:
https://www.youtube.com/watch?v=V9i8qqYHvqA
As for real life: while it would be amazing to have a second thumb semi-permanently, I would prefer it to be pure mechanical contraption without any running software.