You can create Authorizations via Web API by first creating a user (with e-mail) and then the authoration-creation will trigger a default Nuki invite e-mail for the Nuki device. (see Web API Example: Create a key for your Nuki Smart Lock)
And yes, creating a Keypad code (as Operto does) is easier, as you can set the code yourself (if not already used) and then can send it out via the channel of your choice.
@Stefan The idea suggested by @Johannes_De_Silentio would be the best workaround (though not easy to do without knowledge of APIs).
I am not sure if this woule be feasable for you.