Has anyone looked at creating a MailChimp integration? They have a full API that might allow MODx to manage subscribe/unsubscribe/update info.
Of all the commercial Mailing List sites out there, MailChimp seems like it might be a good pairing since they offer a free account with up to 500 list subscribers, but still let you expand if needed. Plus they too integrate with a dozen other sites out there like Freshbooks,
Plus they have code examples that might cut the work in half.
http://www.mailchimp.com/api/examples/