There is a parameter in opendocumet url called sReportName. For details, check the opendocument help document http://help.sap.com/businessobject/product_guides/boexir4/en/xi4_opendocument_en.pdf
If you need to pass parameter from one tab to another without refreshing the entire webi document, raise an idea in SAP IdeaPlace. I believe already a similar idea is in place, you can vote it up.