Hi,
another idea:
1 Create a variant for the variable screen with the used combination.
2 Another idea is to store the changes somewhere with Material, Customer, User, Timestamp and use a pre-query to read from this storage to fill Material, Customer variables. The storage might be a DSO; using the audit feature in InfoCubes you would get User, Timestamp fields for free. Using a planning enabled DSO as a storage for Material, Customer, User, Timestamp one might even be able to mark some combination the user wants to be included as his/her 'favorites'.
For point 2 you need release 7.40.
Regards,
Gregor