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,
This reviews how to create a workflow to send SDFC data to Marketo.
Here is the documentation regarding Marketo 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