The last thing I heard about it (and which seems to be corroberated by the story on this
site (in Dutch)) is that the Brussels Government protected the entire interior, objects,the garden,...
Some of the heirs challenged this decision with the conseil d'etat/raad van state.
Now I tried checking the conseil's d'etat website
and I think I found the arrest
in question (though I'm not sure). From what little French I understand I believe the heirs appeal was rejected.
So I believe that the interior is protected now. I kinda doubt btw that UNESCO would inscribe this property if it wasn't fully protected...