Ability to turn off all base mapping in flex viewer 2.5

Hello.

I have a request from a user that would like to be able to switch all base mapping off completely. In order to view one of the dynamic layers on its own.
Is there a button or widget I can add to the viewer to turn all layers shown in my map switcher off completely so that I just get white space?
I am using version 2.5 of the viewer, ArcGIS Server 10 and compiled code.

I know it is a bit of a strange request but if anyone can help me I would appreciate it.

Harriet