OK, I found the solution:

WITH MEMBER [Measures].[Active Quantity] AS SUM

(

  {

    FILTER

    (

      DESCENDANTS

      (

        [Location Entity].[Geography].CurrentMember,

        [Location Entity].[Geography].[Partner]

      )

      , [Partner].CurrentMember.Properties("Is Active")

    )

  }

  , [Measures].[Quantity]

)

SELECT

[Time].[YMWD].[Year].&[2006] ON 0,

NON EMPTY

(

  DRILLDOWNMEMBER

  (

    DRILLDOWNMEMBER

    (

      [Location Entity].[Geography].[Region]

      ,[Location Entity].[Geography].[Region].&[Asia]

    )

    ,[Location Entity].[Geography].[Country].&[HK]

  )

) ON 1

FROM [Cube]

WHERE ([Measures].[Active Quantity])

To be more precise, this is what I get with the following MDX query:

WITH MEMBER [Measures].[Active Quantity]

as 'IIF(([Partner].CurrentMember.Properties("Is Active") or [Partner].CurrentMember.Properties("Is Active") = null)

and not isempty([Measures].[Quantity]), [Measures].[Quantity], null)'

MEMBER [Measures].[Inactive...