That’s some nifty work, David. I had suggested something similar, whereby heatpumpmonitor.org could host it’s own instance of the My Heatpump app, and pull the data it needs from the individual servers. It seems you found some of the reasons that make that difficult to do, so I think I’m glad I skipped that step.
I also wanted to pull data directly from each instance so that I could calculate SCOP. I started by scraping the config from the HTML of the app page, then Trystan added a special page for me: getconfig. This gives you all the feeds that the app defines, but none of the others.
The primary server could be running a proxy, that loads the config, map the feeds and forwards the data to the webserver, bypassing any issues with https and cors.