Is there a way to send custom data to Mailchimp's API so it can be included in a campaign?

Can I send data (think, different customer fields that would otherwise be sent from inside a Rails mailer) to Mailchimp's API that will be included inside an email (campaign)?

You don't want campaigns, you want transactional emails. Mailchimp supports this through their acquisition of Mandrill: https://kb.mailchimp.com/mandrill/about-transactional-email