Sorry to hear that you are having a tough time finding info in this regard. In any case, the ReportViewer web Part in SharePoint mode supports the IFilterValues interface. You custom web part should implement this interface to supply parameters to the ReportViewer Web Part. In SharePoint Design UI it appears as the "Get Report Parameters From" option in the "Connections" menu that you can use to connect to your custom web part.
Additionally, we are working on documenting the supported Interfaces for the ReportViewer web part. Current plan is to do in the next / upcoming SQL Server 2008 R2 timeframe.