Not sure of a way to do it via an expression in the equation but I have been successful with having two or more equations driving the same dimension but only one of those equations being active for any given configuration.
In the equation dialog box you should see a Configuration button, there you can select the configs you want to assign the equations too and then you can just use the active check box for each equation to selectively set some equations to active/inactive per each configuration.
It does work, but it does get very confusing if you have many configurations and many equations.
Sam