I’ve implemented a V2 which removes the PDAs and uses a TLV-based approach. A list of field authorities are now placed inside the same account as the metadata via a second TLV struct. This allows for much easier composability, no longer having to pass extra accounts (code).
This also matches the UpdateField parameters types and accounts with Token Metadata Interface’s. @joncinque perhaps the Token Metadata Interface’s spec could loosen what is meant by update_authority in the UpdateField function? With TLVs it seems we can actually handle a much broader set of functionality / make this a bit more powerful without having to change any code.