Can I make an App that can show a person their own medical data? i.e, user-provisioned access.
A big tricky part is understanding all the different health systems that have part of the patient's record. Typically speaking you can scrape all health system FHIR access point's and perform some geo matching to offer the ones they likely have seen. From there you do the Oauth2 dance with each health system where the patient authenticates (if they remember their login) and your app gets a token good for a certain time period after which the patient has to log in again.
The advantage of Metriport's approach is that they are getting a hook into the vendor operated HIEs. The patient doesn't have to remember/select which health care systems that have records for them since the VOHIEs have all that. The big hurdle is managing some authentication on behalf of the patient to a third party that they don't have a direct relationship to, the VOHIE. I suppose the VOHIE can pass the patient off to one of the member health systems and do the same Oauth dance but instead of just getting one health systems data, you get the whole enchilada.
The evil part of the operation is that now Metriport has proxy access to the data and eventually will get hacked and bought by private equity that will sell the data to TransEquirian Insurance Score agencies.