I'm not sure when this became available, but it's definitely available in Analysis for Office 1.4 SP4...
The SAPExecuteCommand API function now accepts a parameter value of ShowPrompts that displays the prompts dialog for a given datasource alias (if variables are not merged), or for all datasources...
We've been asking for it on SCN, and in Idea Place, and now it's here...
If variables are not merged, you can call the prompts dialog for a particular Datasource:
Application.Run("SAPExecuteCommand", "ShowPrompts", "DS_1")
If variables are merged, or to call prompts for all datasources..
Application.Run("SAPExecuteCommand", "ShowPrompts", "ALL")