This sounds well-intentioned, but here's a suggestion that I really hope you consider: do the simplest possible thing and make your app spit out a dumb HTML fil

This sounds well-intentioned, but here's a suggestion that I really hope you consider: do the simplest possible thing and make your app spit out a dumb HTML file that represents the patient's dead tree file (where the output is either in a fixed form, or it's marked up with something like microformats). Your app should operate on that, and not JSON. Now the rule becomes, "Even if they don't have the app, they can share the data".