Daniel, I am assessing the use of Tableau on top of SAP BW and quickly identified this as a pretty major issue. Were you able to get around this with any easier solution? I am using desktop 8.2 beta, but it seems to have this limitation too.
Off the top of my head, SAP BW based solutions I can think of would be a user exit variable, so you could create say a DSO that held User ID and the value(s) for a the characteristic you wanted to limit, or perhaps an authorization variable. I just created a similar question as I really just want to prompt the user first or provide a value from Sugar CRM, which we would like to integrate if possible.
For now, that is the best solution. For not too detailed analyses, it actually works quite fast.
In the SAP BW we have everything configured and worked out, but of course we cannot access the Permissions from Tableau. Also if you use Datablending, the second datasource cannot be a Cube as well, so we need to export this information to Oracle/mysql.
After talking to tableau, they mentioned that this might be in cards somewhen.
On the other hand, they mentioned that with Kerberos, which might be implemented soon, this question might be obsolete, because once you are authenticated, the ticket will contain the user information.
As it is now, we have setup SSO from the SAP Portal to the Tableau Server, but for the Tableau Server to the SAP BW communication, we use a embedded password. So that reports are "Single Sign on".
So I am looking forward to the future of SAP BW and Tableau.