@axelchalon mentioned:

@parity/qr-signer returns us the address and chainId when scanning the account, so this can be done. However I think it might be easier to simply show the Signer account in the accounts list if we're connected to the right chain (we might want to show a warning at one point if we import a Signer account for a chainId other than the chain we're currently connected to -- otherwise the user might be surprised not to see their imported signer account show up in the accounts list after completing the import).