This has been something I have been curious to see if it would work. The three foot bucket was a no-go the ears did not have the height for the coupler but the 12in was good to go both pin holes line up. The only draw back is you need two rear pins, the dipper pin is too fat.
Interesting idea, but what is the advantage of the reversed bucket. I see there will be an issue with dumping the contents, which you could overcome by building a new coupler to allow the rotation back. Not sure its worth the trouble as the shovel type machines went away for a reason. Someone else may have some ideas and hopefully they will post them.
The idea is for squaring up a hole, cleaning up the bottom or like here at home transplanting 5 foot or so oaks to better spots on the property without hurting them on the boom or dipper. It would have been nice if the three foot had worked. At height there is still enough dump angel to load a truck with no problem.
We have used a bucket on backwards on mini hoes many times for underpinning existing walls that we had to excavate next to when adding on to buildings. We would dig a bucket width under the wall and foundation to the desired depth, skip two bucket widths and repeat all the way down the wall. As we dug each one we would pour a 300 PSI(1 bag mix) slurry mix back in. After the slurry set up we would go back and repeat next to the cuts backfilling the same way. Then the third time leaving the entire wall underpinned with slurry mix. After turning the bucket around we could dig thru the protruding slurry mix and have a straight walled excavation. These were always engineered by a professional engineer. We would take elevation readings on the building and monitored them for settling. Knock on wood but I never had one move at all.