Altering SSAS writeback behavior?
Writeback is always written to the leaf level of your dimension, and has to be because of the structure of your writeback table (where the difference between the original value and the new value is stored).

Have a look at the writeback tables in your data source and you'll see it contains fields for all your measures, and the key of all your dimensions, so the change always needs to be distributed to that level.

From the documentation on write-enabled partitions


  Changes to leaf and nonleaf cells are handled differently. A leaf cell
  represents an intersection of a measure and a leaf member from every
  dimension referenced by the measure group. The value of a leaf cell is
  taken directly from the fact table, and cannot be divided further by
  drilling down. If a cube or any partition is write-enabled, changes
  can be made to a leaf cell. Changes can be made to a nonleaf cell only
  if the client application provides a way of distributing the changes
  among the leaf cells that make up the nonleaf cell.


The only thing you can really influence is how the changes are distributed to the leaf level cells when you write back to another attribute. 
This is documented in the UPDATE CUBE

You basically have 4 options:


USE_EQUAL_ALLOCATION   
USE_EQUAL_INCREMENT   
USE_WEIGHTED_ALLOCATION [ BY Weight_Expression]   
USE_WEIGHTED_INCREMENT [ BY Weight_Expression]  


I'm not sure how well Excel allows you to pick an allocation method however.