One work around we used in past is to use Marketo SOAP API calls from SFDC workflow on this new opportunity custom object.
Pros:
1> Some times the custom object data sync can get backlogged if your Marketo-SFDC pipe is clogged due to mass update etc. Some times business requirements dictate immediate sync of custom object data. If you go the API route, you remove this possibility of delay.
2> You can have more flexibility of custom validations, and data manipulation, business logic etc
Cons:
1> You will need some development efforts that need expetise in Marketo + SFDC + API programming.
Hope this helps. Please feel free to reach out if I can help in any way...
Rajesh Talele