First, let me say how delighted I am by your attention (as your thought leadership in all things FHIR have been an inspiration to many).
Indeed, as but one of he "many",I am now drafting a White Paper to describe the work that my small company is doing in this specific area (and, of course, we'd be honored to have you review its first draft). The core idea involves more sophisticated (enterprise-to-military-grade) IAM and CA techniques front-ending a blockchained repository of fine-grained patient permissions.
Anyway, I'll look froward to sharing these ideas in hopes of your comments soon.
`till then, all the best