May be you could map the sharing info from SFDC as a custom object in Marketo. If so, from this, you will be able to use the custom object in filters. If this can be done, the hard point might be that the same object will show on lead, contacts and accounts. And you will be able to map only one of them.
I think you might be right on that issue, but I'll explore this. Thanks for the idea, Greg.