Why don't you assign the BU to filter to a session variable and then use the session variable to filter DIM and FACT?

So you would have:

- User to BU table

e.g.

TABLE_USR_BU

--------------

USER1 BU1

USER2 BU1

USER2 BU2

- BU to list of filters to apply table

e.g.

TABLE_BU_FILTER

----------------------

BU1 "Business Unit ABC"

BU2 "Business Unit CDE"

create a variable with the SQL like

select distinct TEXT_TO_FILTER from TABLE_BU_FILTER join TABLE_USR_BU on TABLE_BU_FILTER.BU=TABLE_USR_BU.BU where TABLE_USR_BU.USER=:USER

This will set into your variable the list of BU filters for a certain user and you can use it in your filtering