Report prompt default with SQL

Hi,

We are creating a report on opportunities that should by default filter on opportunities for the user's business unit (custom picklist), but the user must be able to override the filter and choose a different business unit. In a dashboard prompt I can easily do this by creating an sql statement as default value. The user's bu is defaulted in the prompt, and the user can open the drop-down list and select a dfiferent bu. If I try this in a report prompt however, only the user's bu is displayed and the user cannot select a different one. 

How can I achieve this in a report?