Last year in February I brought this up:
...and it was said that there would be further development of the API to support custom fields. I want to integrate an external database with constant contact and use custom fields so the author can use the custom name of the field. Has there been any development on making this possible? Will there be?
We have not made any progress on implementing the new, more flexible custom field functionality available in the product UI. However it is work that we are planning to do this year as part of a larger effort to bring all the new contact management functionality to our API.