I'm still puzzled as to how banks think they're going to do this.
Why is this necessarily a problem? I think the hard forks in both BTC and ETH have shown that there is an inherent "appeals" process no matter what. Seems like you can separate the appeals process from the underlying settlement and record keeping technology. New blockchain providers just need to represent what the ultimate appeals process is for each implementation (instead of hashing power, could easily be any of a dozen other ideas, including recourse to the existing judicial system which has what I'd call a net-neutral set of advantages and disadvantages).
The controversy is over which fork gets to be called Bitcoin.