I've taken a look at this and what's going on in wp-user.php & wp-login.php. There's no proper API which allows an explicit registration which properties are in use. 

We could expose an action 'mollom_registration_extra_fields_info' and ask anyone who implements a custom registration form to implement it, if they want to pass the extra data to Mollom. But then again, I think that would be overkill. It's not our job to push such functionality, but be able to pull data from the system.

So, I agree with B.