Issue Resolution A workaround that can allow for this to happen is to set up a Marketo custom object that mirrors the SFDC object, then use Apex triggers in SFDC to push the information for the SFDC object to the Marketo object - https://developer.salesforce.com/page/Creating_REST_APIs_using_Apex_REST.
In essence, this maps the SFDC object to the Marketo one, and any changes made on the SFDC side will get pushed to Marketo, and can then be triggered off of in your campaigns.
Who This Solution Applies To SFDC integrations with Salesforce custom objects