Our suggested workaround for being able to trigger off updates to SFDC Custom Objects is to actually create a Marketo Custom Object, and use a SFDC Workflow to copy the data from the SFDC Custom Object to the Marketo Custom Object with REST API.
Using this SDFC Documentation,
https://developer.salesforce.com/page/Creating_REST_APIs_using_Apex_REST
This reviews how to create a workflow to send SDFC data to Marketo.
Here is the documentation regarding Marketo API
http://developers.marketo.com/rest-api/
If you create a SFDC Custom Object, and then build a workflow to copy that object data to a Marketo Custom Object, then you would be able to trigger off the updates to the Marketo Custom Object.
Who This Solution Applies To
Clients with SFDC and Marketo Custom Objects