Hey @EarthAdam I was considering the same scenario and had some ideas here: Multiplatform - One Account? - #5 by novabyte
There is still the concern that the user has to have access to the current platform to register a new one though. Having an intermediary like email or facebook could do the trick. So if I lose my existing platform, you could send the account code to email or login as facebook to retrieve it, before registering the new platform.
Personally, I’m getting rid of the idea of ‘device login’. Too many issues can happen, and edge cases to deal with.