You are right, Marketo cannot see the SFDC Opportunity product table and it cannot even be sync'ed as a custom object.
Give a look to this : Marketo / salesforce integration needs a serious overhaul, don't you "sync"? and vote here : Map opportunity products to use them in opportunity filters and triggers
One way to work around this is to use SFDC assets and map them as custom objects.
In the SFDC app exchange, there are some free apps that can automatically convert won oppies into assets and products.