I checked in with our product team on the status of adding support for user-defined custom fields to the V2 API. This is something that we are still doing some research on the best method for implementing. At this time we do not have a planned release date for this, as we want to make sure that we provide the best implementation that we can.
If you are interested in staying up to date on upcoming changes and features for our API, I would suggest subscribing to our developer newsletter at http://developer.constantcontact.com/. It is a monthly newsletter that contains updates and information about features, as well as other useful content related to our API.