I would create a parallel calc that says:
IF (whatever condition I don't want to display for [my original field]) then null else [my original field] end
Then display that calc field instead of the original field.
Not to jump into the middle of anything, but I think your syntax is a bit off Joe. Shouldn't it be:
IF (whatever condition I don't want to display for [my original field]) = [my original field] THEN null ELSE [my original field] END
What's in parentheses isn't code. It's just a thought process.
Actually, what goes in the parentheses might not actually have anything to do with the value of what I want to set. Maybe I want nulls on weekends, or nulls when European profits are negative. The point is to set the actual display value to null when I don't want to see it, whether that conditions occurs in the field I'm displaying, or occurs somewhere else.
Perhaps over simplifying, but just shooting from the hip. Couldn't you just create a filter on your dual axis measures and set the minimum to filter out those results? Does that make sense?