Hi Melissa, I'm not sure if you can set this smart campaign in Marketo (I think that "added to Opportunity" trigger will give you all contact roles added to an opportunity, and I haven't found other trigger that deals with opportunity creation).
However, I found this app in SFDC appexchange that counts and validates contact roles within the opportunity:
https://appexchange.salesforce.com/listingDetail?listingId=a0N300000025Vs1EAE
you can have your SFDC admin test it and see if it fits your needs. You can create a workflow in SFDC that sends an email whenever an opportunity is created with "Number of contact roles" = 0.
I hope that helps
Itay