CCK based Signup (Signup as a cck extension)

I think signup could benefit a lot from being an extension on top of cck, rather than just using it.

Basically, signup would store the signed up users in a multi value user reference field, and allow other fields to be included for the signup.

Allowing node owners to adjust the users would be left to the cck permissions.

Pros
Much more flexible: any field could be used.
More modular: Any cck field could be used as a secondary field, and modules that pull data from cck could interact with the data (such as Node Access Node Reference, which is a very important use case for me).

Cons
More complex to set up
Would need a more complex migration path (probably by automatically creating the needed cck fields)

CCK 3.0 (currently in svn only) has support for multi fields, allowing multiple fields to hold parallel data. This could be used as the primary method for mapping the users to extra signup fields.
Currently multi fields are configured via groups, this takes care of most of the interface. The signup module would add an option to select the multifield group, and either requiring the first field to be a user reference or asking the user what field is the ur.

I am willing to work on this, time permitting.