Did you find a solution for this question? There is a longer discussion thread related to SAP BW located here: Tableau + SAP BW Integration. Problems, limitations, disasters. that may be helpful. Unfortunately, there are not a lot of users out there with SAP BW experience from my background here but you might look at some of these resources - Understanding Functional Differences Between OLAP and Relational Data Source Connections | Tableau Software and the posts and references for Microsoft Analysis Services (MSAS) throughout the community.
I hope that helps.