Hi Kara,
I don't think you will be able to add the "Filter Mode" kind of a thing in the filter portlet.
Actually, OBS related lookups are a bit restricted from the application. It was not there in older version, but during our 13.x versions, we faced lot of issues as customers configured these OBS lookups as per their business requirements and then when they upgrade the PPM application, all those customization got corrupted.
Hope that answers your question.
Regards,
Abhisek Dhar