I have searched the forum on this subject and as far as I am aware this is not possible, in Layout, that is. Am I correct? I have tried all the combinations with no success. In fact, having got it to work in View Map, and then gone to Layout, on return even the View Map version flips back to transparency on!
As others have commented, I cannot believe that this could be allowed to persist by Graphisoft. The need to produce a clean elevation is as about as basic as it can get!