I have a similar situation that I'm hoping you can advise on.
I have a drill down group with CTGY_CODE and ACCT. I need to NULL data for 848020 but I still want the drill down to work. Per your instructions, I am using =if(AU_NUM='848020',null(),CTGY_CODE) for CTGY_CODE and =if(AU_NUM='848020',null(),ACCT) for ACCT. and this hides the data when 848020 is selected but it no longer drills down to the ACCT level.
If I change the expression from =if(AU_NUM='848020',null(),CTGY_CODE) to CTGY_CODE, the drill down to ACCT works but it doesn't obscure 848020 data.