The problem with the card linked is that since everything is printed on the card and you only get two updates, it'll be painful keeping it up to date, especially for episodic people as drugs and doses change frequently. For example, out of cycle you may be on nothing, then start a prednisione taper and say verapamil, ramping up the dose to an effective dose and then ramping down as the cycle ends. There will also me no information about what abortives a person may have taken, especially triptans.

Because of this pattern a doctor would still need to confirm the information, which almost defeats the point, especially as the medical details are as provided by the patient and not from a doctor.

Ideally if you've oxygen available then this would avoid the need to go to hospital as even if you live next door to the hospital, by the time you've got there the CH will have taken hold.

I suspect that cards like this are much more useful for patients with a long term, stable condition where they take the same medication for months on end.

The card could be improved, however it would not be cheap. The card could be a link to a call centre / secure web site which holds full medical records, verified by doctors. There would need to be a secure process to identify both the medical person calling and the patient to ensure privacy. And the data would need to be frequently updated.

What would work much better is centralised medical records, all stored electronically...

Seems like there's a simple fix: Write it on a piece of paper and carry it in your wallet. Have a health care provider read it if you're in no position to explain.

Honestly, I'm not sure why an ED wouldn't give you an oxygen mask if you asked for it. Oxygen isn't a narcotic or something for which drug seekers would be looking. From personal experience, the oxygen you get in hospitals will be delivered at 10 LPM, which, at least for me, is ineffective in knocking back the headaches. Maybe you could talk to your doctor and get he or she to write a paragraph describing why oxygen is used, and you could keep that with you in the event you needed to go to the hospital.
I also don't see why you couldn't just make one of these yourself with an hour or so on microsoft word and a printer.

Good luck,
Isaac