I wouldn't even put any limits on the account (outside of the fact that your address would be an exceptionally unfriendly string.
Since this sign up is through Apple, I assume Hey would have the device ID, and could limit repeat/duplicate/spam sign ups in some way.
I am also assuming that the marginal cost of hosting an account is trivial.
In this situation, _worst case_, someone creates a free account, likes how it works, and then has to pay $99 elsewhere in order to get a reasonable address.
This would kind of be the definition of malicious compliance. And honestly, ya, I would love it.