Epicyclical automatic transmissions installed in automobiles are built up and then inserted into the gearbox. This means that the orbital gear is separate from the gearbox case. The 225 problem is that this gear is integral with the gearbox case so any damage affects the main rotor support structure. A complete redesign where the reduction gearing is separate from, with sufficient room for debris to be ineffective, might be the answer.
You are still going to lose the drive to the main rotor but at least you will have still have some form of control in autorotation as long as you do not shut down the engines.
The gearbox will be bulkier and a bit heavier but it should still be able to fit within the doghouse.