You should be able to use it on a wifi only device: install (sideload?) Signal, enter the phone number you want to register and wait for the timeout, then you'l

You should be able to use it on a wifi only device: install (sideload?) Signal, enter the phone number you want to register and wait for the timeout, then you'll get the verification code via phone call. The decision to not allow exporting the key mitigates certain kind of attacks. I think the wording of the warning is appropriate and not that scary.