To be clear, HL7 is a not-for-profit organization which has created several sets of standards over its history (HL7-V1, V2, V3, etc)--those standards are what you are referring to above.
FHIR, which is what Apple is using, is its own standard, (created/managed through the HL7 organization) and is based on more modern representations of data (JSON, etc). (https://www.hl7.org/fhir/)
To be clear, HL7 is a not-for-profit organization which has created several sets of standards over its history (HL7-V1, V2, V3, etc)--those standards are probably what you are referring to. HL7, as an organization, is very much a leader in this space.
FHIR, which is what Apple is using, is its own standard, (created/managed through the HL7 organization), and addresses most of your concerns (https://www.hl7.org/fhir/)
Yes--HIPAA provides a mechanism for fines to be levied against (most) healthcare organizations that suffer data breaches through the office of civil rights (OCR), with a public reporting of organizations that have been penalized.
Two particularly exciting aspects of this announcement:
(1) Hospitals include more than one EHR vendor (both Epic and Cerner), which shows multi-vendor interoperability.
(2) Apple is using a leading healthcare standards organization (HL7)'s clinical data specification (FHIR, or Fast Healthcare Interoperability Resources), which is a huge win, and will hopefully increase adoption and usage of this open standard by provider organizations.
Though not exactly as you described, something similar was done and published in 2016 (largely leading to this change, since there was not an increase in mortality risk)
One of the main challenges of course in this area is looking at patient outcomes with an incredible amount of potentially confounding variables, since "resident work hours" is just one of the many things that influence patient outcomes at a hospital.
The point is that advanced directives should not be tied to a specific healthcare system. Decentralization (blockchain or otherwise) is an interesting way to make this more accessible.
Practically, advanced directive knowledge requires verbal confirmation with the patient or healthcare proxy; or something like a MOLST (paper).
A more interesting application would be storing health "meta data" - for example, permissions around viewing health data, or focused data like advanced directives ("code status"). For advanced directives, this could be really valuable to guarantee that disparate health care systems that don't share data are aware of your wishes.
FHIR, which is what Apple is using, is its own standard, (created/managed through the HL7 organization) and is based on more modern representations of data (JSON, etc). (https://www.hl7.org/fhir/)