But if it's impossible at the protocol level, then maybe we can look at safeguards at the client level. Maybe clients can (at least have the option of) enforcing check digits for entering naked addresses.
NRS API can be changed without breaking backward compatibility.
Inside can be stored without check digit - as before. And take / give out only with check digit.
NRS & clients must check this, not client only.
First time to use the old and new version of account number together, developers need time to change their apps.