> Android could append the unique app identifier (ie. "com.myapp") to the end of any data to be signed. Then the user can't be tracked between apps. But it also prevents you using 'sim sign in' to sign in to the same service from a web browser and app for example.
I doubt that: simply add two "SIM identities" (which on the mobile phone map to the same SIM card) to the account of the respective service.