Hi Jason,
Custom Object sync is only one way,custom object Data from SFDC will sync into Marketo and can be used as filters/triggers but Marketo doesn't push custom object data to SFDC, like opportunities it's ready only access.
The fields would need to be on your lead and/or contact object in order for the values to be entered into a form and then pushed from Marketo to SFDC.
Rgs,
Cathal