I would could create a couple custom integration events, an inbound one that matches on email address to sync the SFDC Lead/Contact ID to the Eloqua contact record, and then an outbound call that updates the subscription status in salesforce. Put them both into a program fed by contacts created in the last X days.
You also could add a decision rule to make sure the contacts going though don't already have SFDC contact/lead IDs, just to be extra sure you aren't overwriting any existing data.