Innumerable people sync SFDC Custom Objects to Marketo. It's a question of whether the SFDC API user used by Marketo can see the object.
However, although the CO records can be synced to Marketo (+ used in Smart List filters + output in Velocity) you won't be able to trigger on changes to a field on the CO.
Thank you for the reply.
It seems the best course of action would be to create an SFDC automation that changes a field when the status of a CO changes. Would this be the best way to handle this situation?