Output an extra measure only for members of a set

I am outputting a list of locations and a measure from my cube for each of those locations.  That is working fine.  However, based on a parameter I have a list of 1 or more specific locations that I want to have an additional measure that is either calculated, or just 0. These "special" locations are in a named set.

I am basically looking for some kind of CASE statement functionality that accomplishes something like...

WITH

  SET [SpecialSites] as StrToSet(@PickedSites)

MEMBER [Measures].[BonusVal] AS

CASE WHEN [DimSite].[Site].CurrentMember IS IN [SpecialSites]

THEN 100

ELSE 0

END

So, if sites 2 and 5 happen to be in my special subset of sites, when I output my cube for my report, I get

SiteVal BonusVal

120 0

225 100

318 0

419 0

527 100

I know I could do it with a case statement with a fixed set of dimensional members, but I cannot hard code it - I'll never know which one it is since it will be a parameter.  I could easily do it with a traditional stored procedure, but I'd rather do it in a cube.

Thanks!

Mgr, Data Warehouse Svcs