That is a question which can only be answered by individual EMTs who respond to your emergency. They would be using their own device to scan your QR, and every smart device is likely to be different.
If EMTs attend and find you unresponsive they will almost certainly focus on keeping you alive for long enough to reach a hospital. That is their training. The EMTs are unlikely to have time to look for a device which just might hold a hidden QR code pointing to some document they might be able to read, and they would not know they had to somehow activate functions of a smart watch to reveal a QR. In a critical scenario the delay looking for it could be fatal for someone who could otherwise survive.
This might not be what you want to hear, but it is how I see the reality. If you have a DNR directive and want it to be respected in a first contact environment it needs to be something which can be read and authenticated by anyone who can read the physical document, without using technology.