Analysis :: Calculate Members At Certain Level In One Hierarchy As Sum Of Corresponding Members At A Certain Level

Problem setting is a geography dimension with multiple user defined hierarchies in SSAS 2008. 

Ex.: 
Hierarchy 1 (political territory): level 6 --> level 5 --> level 4 --> level 3 (state) --> level 2 --> level 1
Hierarchy 2 (sales territory): level 4 --> level 3 --> level 2 (sales region) --> level 1
...
Hierarchy 9

The relationship between state and sales region is n:1, i.e. one state belongs to exactly one sales region, and one sales region can consists of one or multiple states.  Unfortunatly I can't define this attribute relationsship in the dimension because it would lead to a diamond-shaped relationsship without a user-defined-hierarchy to back it up. So far that isn't much of a problem, user don't drill down from sales region to state. But now I want to define a calculated member that multiplies a measure from the main measure group with another measure from a weighting factor measure group at the state level and above. The granularity attribute of the geography dimension in the dimension usage tab of the weighting factor measuregroup is the state. 

So far what I've got is:

CREATE MEMBER Currentcube.Measures.[weighted measure state and above] AS NULL;
SCOPE (Measures.[weighted measure state and above],
Descendants(geography.[political territory].[all member],3,SELF_AND_BEFORE),
Descendants(geography.[salesterritory].[all member],2,SELF_AND_BEFORE),
... Descendants(geography.[hierarchy 9].[all member],1,SELF_AND_BEFORE)); this = sum(existing(geography.[political territory].state.members), measures.[main measure group measure] * measures.[weighting measure group measure]);END SCOPE;

This works from a functional point of view, but is rather slow when querying any other hierarchy than the political territory hierarchy, because SSAS first goes down from the state level to the key attribute of the geography dimension, and then aggregates from there to the sales region.In other words, I want SSAS to resolve the relationsship (which state belongs to which sales region) through the dimension, and not through the fact, and apply the calculation afterwards. Like some kind of currency conversion, but only from a certain level upwards.