I just tried and on Android even if you prefix a specific schema, e.g. "myapp:nostr:xxx", using the Web Share API chromium instead of switching immediately offers to open the already "nostr:" registered app, but without giving the option to pick a different one:
https://image.nostr.build/224bdae90cc256719a8cce92163aee060e2c62a9ce3a77ac4f34240d282b609a.jpg
The weird thing is that this happens even if I completely change the schema removing the "nostr" part, it seems it has a cache somewhere.
Instead, using a simple href I got a more logic "Failed to launch "myapp:nostr:xxxxx .... because the schema does not have a registered handler". So registering the apps as nostr:npub1n0sturny6w9zn2wwexju3m6asu7zh7jnv2jt2kx6tlmfhs7thq0qnflahe did should solve the problem, but we need to hard-code all the possibilities.
I need to do more tests, I have the remote debug broken right now, so I'm doing it by trial and error.
I don't know if IOS acts differently.